Gas-detecting element and gas-detecting device comprising same

ABSTRACT

A gas-detecting element comprising an oxygen-ion-conductive solid electrolyte substrate, a sensing electrode fixed onto said solid electrolyte substrate and active with a detection object gas and oxygen, and a reference electrode fixed onto said solid electrolyte substrate and active with at least oxygen, for detecting potential difference between the sensing electrode and the reference electrode to determine the concentration of the detection object gas, the sensing electrode and/or the reference electrode being covered by an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, and the electrode-coating layer having a portion bonded to the solid electrolyte substrate directly or via an electrode underlayer made of an oxygen-ion-conductive solid electrolyte.

FIELD OF THE INVENTION

[0001] The present invention relates to a gas-detecting element and agas-detecting device for measuring the concentration of a detectionobject gas in a gas atmosphere, particularly to a gas-detecting elementand a gas-detecting device suitable for directly measuring theconcentration of nitrogen oxides in a combustion exhaust gas emittedfrom automobiles, etc.

BACKGROUND OF THE INVENTION

[0002] So-called gas sensors with high gas selectivity capable ofelectrochemically detecting a particular gas by using solid electrolytesubstrates have recently been proposed actively. Particularly, gassensors capable of measuring the concentration of total NOx in anexhaust gas from automobiles without affected by other gases arestrongly demanded.

[0003] Thus, the inventors previously proposed a mixed-potential-typeNOx sensor comprising an oxygen-ion-conductive zirconia solidelectrolyte operable at high temperatures in JP 9-274011 A. This NOxsensor has a basic structure, which comprises a NOx-sensing electrodeand a reference electrode formed on an opposite or same surface of azirconia solid electrolyte substrate as the NOx-sensing electrode. Inthis NOx sensor, a sensing electrode is, of course, exposed to adetection gas (gas to be detected), and a reference electrode can besimultaneously exposed to a detection gas, if the reference electrode isactive with only oxygen. Because the NOx-sensing electrode is activewith NOx and oxygen, and because the reference electrode is active onlywith oxygen, output (potential difference) can be obtained due to thedifference in chemical potential between both electrodes. Accordingly,the measurement of potential difference between both electrodes leads tothe detection of the NOx concentration in the detection gas.Incidentally, when the reference electrode is also active with NOx, thesame NOx sensitivity can be obtained if isolated from the detection gas.

[0004] At the time of detecting a gas by the sensing electrode of theabove mixed-potential-type NOx sensor, however, NO is subjected toreactions represented by the following formulae (1) and (2):

O₂+4e⁻→2O²⁻  (1), and

2NO+2O²⁻→2NO₂+4e⁻  (2),

[0005] and NO₂ is subjected to reactions represented by the followingformulae (3) and (4):

2O²⁻→O₂+4e⁻  (3), and

2NO₂+4e⁻→2NO+2O²⁻  (4).

[0006] As a result, sensor outputs with NO and NO₂ at the time ofdetecting a gas are just opposite in polarity. When the concentration oftotal NOx is detected in an exhaust gas emitted from vehicles, thecoexistence of NO and NO₂ causes interference if no measure is taken,failing to detect the concentration of total NOx precisely.

[0007] Accordingly, JP 9-274011 A proposes a laminate-type gas-detectingdevice. According to the principle of this laminate-type gas-detectingdevice, oxygen from air is introduced into a gas detection chamber usingan electrochemical oxygen pump. As a result, reducing gases such as HC(hydrocarbons), CO (carbon monoxide), etc. in the detection gas areoxidized to be harmless. Simultaneously, NO in NOx is electrochemicallyconverted to NO₂, so that NOx becomes consisting only of NO₂. After thistreatment for turning a detection gas to contain only one detectionobject gas, the NO₂ concentration is measured from the potentialdifference between the NOx-sensing electrode and the referenceelectrode, thereby determining the concentration of total NOx.

[0008] In such NOx-detecting element or such laminate-type NOxgas-detecting device, its detection performance, namely sensitivity andits stability and response, is particularly governed by the performanceof a sensing electrode. Conventionally reported as the sensingelectrodes of such mixed-potential-type NOx sensors are, for instance,NiCr₂O₄ (SAE Paper No. 961130), Pt—Rh alloys or cermet electrodescomprising Pt—Rh alloys to which a zirconia solid electrolyte is added(JP 11-72476 A). These sensing electrodes have excellent sensitivity.However, further improvement is needed with respect to the stability ofsensitivity of sensing electrodes. For this purpose, it is important toimprove the stability of an electrode material per se, and the bondingstability of interface (electrode interface) between a solid electrolytesubstrate and a sensing electrode. Particularly when metal oxides areused for the electrodes, it has conventionally been difficult to controlthe bonding stability of this electrode interface. This is because thereis generally weak bonding between metal oxides and solid electrolytesubstrates, resulting in the likelihood that peeling and cracking occurin their interface.

[0009] To improve the response of gas detection, it is necessary toreduce the interface impedance of the electrode in the gas sensor. Forthis purpose, increase in an electrode area and the elevation ofoperation temperatures have been investigated. However, in amixed-potential-type sensor, the higher the temperature, the lower thegas sensitivity. In addition, to increase an electrode area, it isnecessary to make a sensor element larger. The increase of the sensorelement deteriorates the uniformity of the temperature distribution ofthe sensor element, resulting in the variation of performance andinstability.

[0010] As described above, though there are materials excellent insensitivity for the mixed-potential-type NOx sensor, further improvementis needed with respect to the stability of sensitivity. Particularlywhen a metal oxide electrode is used as a sensing electrode, there ispoor bonding stability with the solid electrolyte substrate, resultingin the variation of detection performance and decrease in yield.Therefore, it is desired not only to improve interface stability betweenthe sensing electrode and the solid electrolyte substrate, but also toreduce the variation of characteristics that are caused during aproduction process for some reasons. Further, it is desired to improvegas response without making the sensor element larger, and withoutaccompanying decrease in gas sensitivity.

[0011] Though the importance of stability and response of the sensingelectrode has been described above, such characteristics are notrequired only to the sensing electrode. In the case of the NOx sensor,for instance, it is important to improve the stability and response of areference electrode serving as a reference for electrode potential, andan oxygen-sensing electrode for making compensation for oxygenconcentration, etc., because these characteristics also affect theperformance of the NOx sensor.

[0012] In the case of a NOx sensor mounted onto a vehicle, oxygenconcentration in the detection gas widely varies, and thus the influenceof the concentration of coexisting oxygen cannot be neglected. In theNOx sensor of this type, a reference electrode active only with oxygenis disposed in a portion close to the NOx-sensing electrode, and themeasurement of potential between the NOx-sensing electrode and thereference electrode leads to the determination of NOx concentration inthe detection gas. By mounting an oxygen-sensing electrode active withoxygen but inactive with NOx near the NOx-sensing electrode within adetection chamber, by measuring potential difference E₂ between thereference electrode and the oxygen-sensing electrode disposed in an airduct, and potential difference E₁ between the reference electrode andthe NOx-sensing electrode, and by arithmetically treating thesedifference (E₁-E₂), it is possible to compensate the variation of oxygenconcentration. The measurement using such electrode inactive with adetection gas enables high-precision measurement of the concentration ofa detection object gas even with a detection gas such as an exhaust gasfrom automobiles, etc. in which oxygen concentration varies.

[0013] However, if the reference electrode or the oxygen-sensingelectrode becomes considerable active with NOx (exhibits mixedpotential), its influence decreases the precision of NOx detection. Toimprove the sensitivity of the NOx sensor, it is desirable to reduce theactivity of the reference electrode or the oxygen-sensing electrode withNOx. The activity of the reference electrode or the oxygen-sensingelectrode with NOx is presumed to be generated by the contamination ofthese electrodes, vapor deposition from other electrodes evaporatedduring the sintering step, etc. Though the contamination can beprevented by control of the production process, the vapor depositionfrom other electrodes during the sintering step is inevitable because ofthe restrictions of the sintering conditions of substrates, sensingelectrode characteristics, etc. Accordingly, it is important to minimizethe activity of the reference electrode and the oxygen-sensing electrodewith a detection gas such as NOx, etc., during the production andoperation.

[0014] In the laminate-type NOx gas-detecting device, its detectionperformance is affected by the performance of the conversion electrodefor electrochemically converting NO to NO₂ or NO₂ to NO in the NOx.Therefore, the conversion electrode should carry out the desired oxygenpumping. The factors for varying oxygen pumping are the change ofelectric resistance of a conversion electrode per se, the change ofinterface resistance between a conversion electrode and a solidelectrolyte substrate, the change of bulk resistance of a solidelectrolyte per se, etc. Also, the conversion electrode should have notonly an excellent oxygen pumping function, but also excellentperformance of adsorption and desorption of NO for electrochemicallyconverting NO in the NOx to NO₂. From these facts, the conversionelectrode should be excellent in electrochemical stabilitycharacteristics.

[0015] However, it is necessary to further improve the conversionelectrode with respect to stability of sensitivity. For this purpose, itis important to have good bonding stability of interface (electrodeinterface) between the solid electrolyte substrate and the conversionelectrode. By the difference in a sintering shrinkage ratio and athermal expansion coefficient between the solid electrolyte substrateand the conversion electrode, it is difficult to maintain the bondingstability of electrode interface for a long period of time. Particularlywhen the conversion electrode contains a metal oxide, there is weakbonding with the solid electrolyte substrate, making it likely to causepeeling and cracking in the interface.

[0016] Further, when the conversion electrode comes into direct contactwith a strong reducing gas such as HC (hydrocarbons), CO (carbonmonoxide), etc., its performance of adsorption and desorption of NO islikely to be remarkably changed. The laminate-type, gas-detecting deviceof JP 9-274011 A oxidizes a reducing gas such as HC (hydrocarbons), CO(carbon monoxide), etc. in the detection gas to be harmless. However,when this gas-detecting device is used for automobiles, the temperatureof the gas-detecting device is not so elevated at the time of startingan engine that the conversion pump element does not fully work. Thus,when exposed to HC and CO, the adsorption and desorption of NO isremarkably changed.

OBJECT OF THE INVENTION

[0017] Accordingly, an object of the present invention is to provide agas-detecting element and a gas-detecting device excellent in thebonding stability of interface between an electrode and a solidelectrolyte substrate, with the activity of a reference electrode or anoxygen-sensing electrode with a detection object gas suppressed, therebyexhibiting stable sensitivity and excellent response performance.

SUMMARY OF THE INVENTION

[0018] The first gas-detecting element of the present inventioncomprises an oxygen-ion-conductive solid electrolyte substrate, asensing electrode fixed onto the solid electrolyte substrate and activewith a detection object gas and oxygen, and a reference electrode fixedonto the solid electrolyte substrate and active with at least oxygen, todetermine the concentration of the detection object gas from thepotential difference between the sensing electrode and the referenceelectrode, wherein the sensing electrode and/or the reference electrodebeing covered by an electrode-coating layer made of anoxygen-ion-conductive solid electrolyte, the electrode-coating layerhaving a portion bonded to the solid electrolyte substrate directly orvia an electrode underlayer made of an oxygen-ion-conductive solidelectrolyte.

[0019] By covering a sensing electrode with an electrode-coating layermade of an oxygen-ion-conductive solid electrolyte, it is possible toreduce the bonding instability of interface between a solid electrolytesubstrate and the sensing electrode, which is caused by thermal stressdue to the difference in a thermal expansion coefficient between them.Though there is an electrode interface (three-phase interface) only in abonding interface between a sensing electrode and a solid electrolytesubstrate in a conventional gas-detecting element, a bonding interfacebetween a sensing electrode and an electrode-coating layer also servesas an electrode interface in the gas-detecting element of the presentinvention, resulting in drastic increase in an electrode interface area.Accordingly, the electrode impedance can be reduced, resulting inimprovement in gas response.

[0020] By covering a reference electrode with an electrode-coating layermade of an oxygen-ion-conductive solid electrolyte, the stability ofinterface and the gas response are also improved, like the sensingelectrode. When the reference electrode is also exposed to a detectiongas (a gas to be detected), the decrease of interface impedance leads tothe increase of reaction sites of oxygen, as long as oxygenconcentration is sufficiently higher than the concentration of adetection object gas in the detection gas. In this case, becausereaction sites of a low concentration of a detection object gas (forinstance, NOx) are not substantially influenced, the activity of thereference electrode to the detection object gas decreases. Further, theelectrode-coating layer prevents contamination to the referenceelectrode during production processes and use of a sensor, so that thesensitivity of the reference electrode to the detection object gas canbe kept low, resulting in improvement in the precision and stability ofa sensor.

[0021] The second gas-detecting element of the present inventioncomprises an oxygen-ion-conductive solid electrolyte substrate, asensing electrode fixed onto the solid electrolyte substrate and activewith a detection object gas and oxygen, an oxygen-sensing electrodefixed onto the solid electrolyte substrate and active with at leastoxygen, a reference electrode positioned in an atmosphere separated froma detection object atmosphere and active with oxygen, to determine theconcentration of the detection object gas from the difference (E₁−E₂)between a potential difference E₁ between the sensing electrode and thereference electrode and a potential difference E₂ between theoxygen-sensing electrode and the reference electrode, wherein thesensing electrode and/or the oxygen-sensing electrode being covered byan electrode-coating layer made of an oxygen-ion-conductive solidelectrolyte, the electrode-coating layer having a portion bonded to thesolid electrolyte substrate directly or via an electrode underlayer madeof an oxygen-ion-conductive solid electrolyte.

[0022] By covering a sensing electrode and/or an oxygen-sensingelectrode with an electrode-coating layer, the bonding stability ofinterface between a sensing electrode and/or an oxygen-sensing electrodeand a solid electrolyte substrate, and gas response are improved. Also,the oxygen-sensing electrode exposed to a detection gas has loweredactivity with the detection object gas, and it is possible to preventthe oxygen-sensing electrode from having activity with a detectionobject gas by contamination, thereby improving the precision andstability of the detection element.

[0023] Preferred examples of the gas-detecting element of the presentinvention are as follows:

[0024] (1) The electrode-coating layer covering the sensing electrodeand/or the reference electrode is in the form in which a detection gascan reach a three-phase interface between each electrode and the solidelectrolyte substrate, the electrode underlayer or the electrode-coatinglayer.

[0025] (2) At least one of the sensing electrode, the referenceelectrode and the oxygen-sensing electrode is fixed onto the solidelectrolyte substrate via an electric insulating layer.

[0026] (3) At least one of the sensing electrode, the referenceelectrode and the oxygen-sensing electrode is fixed in a recess formedon the solid electrolyte substrate.

[0027] (4) The electrode-coating layer covering the sensing electrodehas a porosity of 10-50%.

[0028] (5) The electrode-coating layer covering the sensing electrodehas an average thickness of 3-20 μm.

[0029] (6) The electrode-coating layer covering the reference electrodeor the oxygen-sensing electrode has a porosity of 0-50%.

[0030] (7) The electrode-coating layer covering the reference electrodeor the oxygen-sensing electrode has an average thickness of 1-20 μm.

[0031] (8) The electrode-coating layer covering at least one of thesensing electrode, the reference electrode and the oxygen-sensingelectrode has an average thickness of 5-100 μm, and theelectrode-coating layer is provided with gas-diffusing pores, a ratio(Sh/Se) of the total opening area (Sh) of the gas-diffusing pores to thearea (Se) of the sensing electrode being 0.05-0.28.

[0032] (9) An upper surface of at least one electrode of the referenceelectrode and the oxygen-sensing electrode exposed to a detection gas iscovered by a dense electrode-coating layer, and part of side surfaces ofthe electrode is exposed.

[0033] (10) A plurality of sensing electrodes are formed via theelectrode-coating layer covering the sensing electrode.

[0034] (11) The electrode-coating layer covering at least one of thesensing electrode, the reference electrode and the oxygen-sensingelectrode is made of a zirconia solid electrolyte containing at leastone selected from the group consisting of yttria, ceria, magnesia andscandia as a stabilizer.

[0035] (12) The electrode-coating layer covering the sensing electrodecontains a precious metal active with the detection object gas andoxygen.

[0036] (13) The electrode-coating layer covering at least one of thesensing electrode, the reference electrode and the oxygen-sensingelectrode contains a precious metal inactive with the detection objectgas but active with oxygen.

[0037] (14) The electrode underlayer is made of a zirconia solidelectrolyte containing at least one selected from the group consistingof yttria, ceria, magnesia and scandia as a stabilizer.

[0038] (15) The sensing electrode is made of a metal oxide and/or aprecious metal active with a detection object gas and oxygen.

[0039] (16) The detection object gas is any of nitrogen oxides,hydrocarbon, carbon monoxide or ammonia.

[0040] The first gas-detecting device of the present invention comprises(a) a gas-measuring chamber defined by first and secondoxygen-ion-conductive solid electrolyte substrates disposed with apredetermined gap; (b) a gas inlet so that a detection gas flows intothe gas-measuring chamber with a predetermined gas diffusion resistance;(c) a gas-detecting element comprising a sensing electrode fixed ontothe first solid electrolyte substrate such that it is exposed to anatmosphere in the gas-measuring chamber, and active with a detectionobject gas and oxygen, and a reference electrode fixed onto the firstsolid electrolyte substrate and active with at least oxygen; (d) adetection-object-gas-converting pump element comprising (i) adetection-object-gas-converting electrode fixed onto the second solidelectrolyte substrate such that it is exposed to an atmosphere in thegas-measuring chamber, and active with a detection object gas andoxygen, and (ii) a detection-object-gas-converting counter electrodefixed onto the second solid electrolyte substrate such that it isexposed to an atmosphere containing oxygen and/or an oxide gas, andactive with oxygen, which can select the oxidation or reduction of adetection object gas depending on conditions; (e) a means for measuringthe potential difference between the sensing electrode and the referenceelectrode; and (f) a voltage-applying means for driving the conversionpump element, to detect the potential difference between the sensingelectrode and the reference electrode while applying predeterminedvoltage to the conversion pump element, thereby determining theconcentration of the detection object gas in the detection gas, whereinthe sensing electrode being covered by an electrode-coating layer madeof an oxygen-ion-conductive solid electrolyte, and the electrode-coatinglayer having a portion bonded to the first solid electrolyte substratedirectly or via an electrode underlayer made of an oxygen-ion-conductivesolid electrolyte.

[0041] The detection object gas suitable for the above gas-detectingdevice is NOx.

[0042] The second gas-detecting device of the present inventioncomprises (a) a gas-measuring chamber defined by first and secondoxygen-ion-conductive solid electrolyte substrates disposed with apredetermined gap; (b) a gas inlet provided in the gas-measuring chambersuch that a detection gas flows into the gas-measuring chamber with apredetermined gas diffusion resistance; (c) a gas-detecting elementcomprising a sensing electrode fixed onto the first solid electrolytesubstrate such that it is exposed to an atmosphere in the gas-measuringchamber, and active with a detection object gas and oxygen, and areference electrode fixed onto the first solid electrolyte substrate andactive with at least oxygen; and (d) a detection-object-gas-convertingpump element comprising (i) a detection-object-gas-converting electrodefixed onto the second solid electrolyte substrate such that it isexposed to an atmosphere in the gas-measuring chamber, and active with adetection object gas and oxygen, (ii) a detection-object-gas-convertingcounter electrode fixed onto the second solid electrolyte substrate suchthat it is exposed to an atmosphere containing oxygen and/or an oxidegas, and active with oxygen, which can select the oxidation or reductionof a detection object gas depending on conditions; (e) a means formeasuring the potential difference between the sensing electrode and thereference electrode; and (f) a voltage-applying means for driving theconversion pump element, thereby detecting the potential differencebetween the sensing electrode and the reference electrode while applyingpredetermined voltage to the conversion pump element, to determine theconcentration of the detection object gas in the detection gas; theelectrode for converting the detection object gas being covered by anelectrode-coating layer made of an oxygen-ion-conductive solidelectrolyte, through which the detection object gas can reach to theelectrode; and the electrode-coating layer having a portion bonded tothe second solid electrolyte substrate directly or via an electrodeunderlayer made of a solid electrolyte.

[0043] Preferred examples of the gas-detecting device of the presentinvention are as follows:

[0044] (1) The electrode-coating layer covering thedetection-object-gas-converting electrode is preferably in such a formthat a detection gas can reach a three-phase interface of the solidelectrolyte substrate, the electrode underlayer or the electrode-coatinglayer and each electrode.

[0045] (2) The electrode-coating layer is constituted by a porous solidelectrolyte film layer having pores through which the detection objectgas can be diffused, the porous solid electrolyte film layer having aporosity of 10-50% and an average thickness of 3-20 μm.

[0046] (3) The electrode-coating layer covering thedetection-object-gas-converting electrode is made of a zirconia solidelectrolyte containing as a stabilizer at least one selected from thegroup consisting of yttria, ceria, magnesia and scandia.

[0047] (4) The electrode-coating layer covering thedetection-object-gas-converting electrode comprises (a) at least oneprecious metal selected from the group consisting of platinum, rhodium,iridium, gold and alloys containing these metals, and/or (b) at leastone metal oxide selected from the group consisting of Cr₂O₃, NiO,NiCr₂O₄, MgCr₂O₄ and FeCr₂O₄ in a range of 1-50% by volume based on 100%by volume of the solid electrolyte.

[0048] (5) The electrode underlayer is made of a zirconia solidelectrolyte containing as a stabilizer at least one selected from thegroup consisting of yttria, ceria, magnesia and scandia.

[0049] (6) Said electrode underlayer comprises (a) at least one preciousmetal selected from the group consisting of platinum, rhodium, iridium,gold and alloys containing these metals, and/or (b) at least one metaloxide selected from the group consisting of Cr₂O₃, NiO, NiCr₂O₄, MgCr₂O₄and FeCr₂O₄ in a range of 0.1-20% by volume based on 100% by volume ofthe solid electrolyte.

[0050] (7) The detection-object-gas-converting electrode is made of atleast one precious metal selected from the group consisting of platinum,rhodium, iridium, gold and alloys containing these metals.

[0051] (8) The detection-object-gas-converting electrode and a layer forcoating this electrode are made of a zirconia solid electrolytecontaining the same stabilizer, the stabilizer being at least oneselected from the group consisting of yttria, ceria, magnesia andscandia.

[0052] (9) The gas-detecting device further comprises a means forheating at least the gas-detecting element and thedetection-object-gas-converting pump element to a predeterminedtemperature.

[0053] (10) The detection object gas is any of a nitrogen oxide gas, ahydrocarbon gas, a carbon monoxide gas and an ammonia gas.

[0054] (11) The detection object gas is nitrogen oxide, and theoxidation reaction of NO to NO₂ or the reduction reaction of NO₂ to NOin the detection gas by the conversion pump element can be selecteddepending on conditions.

[0055] (12) The above gas-detecting element is the gas-detecting elementof the present invention.

[0056] (13) When the concentration of a reducing detection object gas ismeasured by the gas-detecting device of the present invention, at leasta sensing electrode is exposed to an atmosphere containing 0.1% byvolume or more of oxygen, to measure potential difference between thesensing electrode and the reference electrode.

[0057] The electrode-coating layer prevents the direct contact of theconversion electrode with a detection gas. By driving a conversion pumpelement and/or a gas-treating pump element, oxygen is pumped into thegas-measuring chamber, in which a reducing gas in the detection gas canbe oxidized. Particularly in the case of driving with a conversionelectrode of a conversion pump element as an anode, the formation of theelectrode-coating layer increases oxidation efficiently of a reducinggas by oxygen pumped from the conversion electrode. Accordingly, adetection gas containing a high concentration of a reducing gas is notdirectly contacted with the conversion electrode, thereby suppressingthe remarkable change of adsorption and desorption performance of NO.

[0058] Because bonding interface between the conversion electrode andthe electrode-coating layer serves as electrode interface, an electrodeinterface area can be drastically increased, thereby suppressing thevariation of interface impedance between the conversion electrode andthe solid electrolyte substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059]FIG. 1 is a schematic cross-sectional view showing a gas-detectingelement according to one embodiment of the present invention;

[0060]FIG. 2 is a schematic cross-sectional view showing a gas-detectingelement according to another embodiment of the present invention;

[0061]FIG. 3 is a schematic cross-sectional view showing a gas-detectingelement according to a further embodiment of the present invention;

[0062]FIG. 4 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0063]FIG. 5 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0064]FIG. 6 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0065]FIG. 7 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0066]FIG. 8 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0067]FIG. 9 is a schematic cross-sectional view showing a gas-detectingelement according to a still further embodiment of the presentinvention;

[0068]FIG. 10 is a schematic cross-sectional view showing agas-detecting element according to a still further embodiment of thepresent invention;

[0069]FIG. 11 is a schematic cross-sectional view showing agas-detecting element according to a still further embodiment of thepresent invention;

[0070]FIG. 12 is a schematic cross-sectional view showing agas-detecting element according to a still further embodiment of thepresent invention;

[0071]FIG. 13 is a schematic cross-sectional view showing agas-detecting element according to a still further embodiment of thepresent invention;

[0072]FIG. 14 is a schematic cross-sectional view showing agas-detecting device according to one embodiment of the presentinvention;

[0073]FIG. 15 is a schematic cross-sectional view showing agas-detecting device according to another embodiment of the presentinvention;

[0074]FIG. 16 is a plan view showing the gas-detecting device of FIG.15;

[0075]FIG. 17 is a schematic cross-sectional view showing agas-detecting device according to a further embodiment of the presentinvention;

[0076]FIG. 18 is a schematic cross-sectional view showing agas-detecting device according to a still further embodiment of thepresent invention;

[0077]FIG. 19 is a schematic cross-sectional view showing agas-detecting device according to a still further embodiment of thepresent invention;

[0078]FIG. 20 is an exploded view of the gas-detecting device of FIG.18;

[0079]FIG. 21 is a schematic cross-sectional view showing agas-detecting device according to a still further embodiment of thepresent invention; and

[0080]FIG. 22 is a schematic cross-sectional view showing agas-detecting device according to a still further embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] [1] Gas-detecting Element

[0082] (A) One Example of Gas-detecting Element with Coated SensingElectrode

[0083] Referring to FIG. 1, the basic structure of the gas-detectingelement of the present invention comprising a sensing electrode coveredby an electrode-coating layer will be explained. In the gas-detectingelement of the present invention shown in FIG. 1, anoxygen-ion-conductive solid electrolyte substrate 1 has one surface, onwhich a sensing electrode 5 active with a detection object gas andoxygen is formed, and the other surface, on which a reference electrode7 is formed. In the present invention, the sensing electrode 5 and/orthe reference electrode 7 is covered by an electrode-coating layer of anoxygen-ion-conductive solid electrolyte. The electrode-coating layer isin such a form that a detection gas can reach a three-phase interface ofa solid electrolyte substrate 1, an electrode underlayer or anelectrode-coating layer and each electrode. In this example, anelectrode-coating layer 11 made of a porous oxygen-ion-conductive solidelectrolyte is laminated on the sensing electrode 5, and theelectrode-coating layer 11 has a portion bonded directly to the solidelectrolyte substrate 1. Also, in this example, the sensing electrode 5is exposed to a detection gas atmosphere, which contains a detectionobject gas, and the reference electrode 7 is exposed to a reference gasatmosphere, for instance, air, to obtain a constant reference potential.

[0084] When the detection object gas is NOx, reactions represented bythe above formulae (1) and (2), and (3) and (4) occur simultaneously onthe sensing electrode 5, generating an equilibrium potential (mixedpotential) on the electrodes. Because oxygen is involved in suchreactions, a detection gas, to which the sensing electrode 5 is exposed,should contain 0.1% by volume or more of oxygen. To achieve rapid gasresponse, the detection gas preferably contains 1% by volume or more ofoxygen.

[0085] (1) Solid Electrolyte Substrate

[0086] The materials of the solid electrolyte substrate 1 are notrestrictive as long as they have oxygen ion conductivity. Theoxygen-ion-conductive materials are preferably zirconia solidelectrolytes containing yttria (Y₂O₃), etc. as stabilizers for improvingchemical stability and mechanical strength. When the amount of yttriaadded is 3-10 mol % based on the total amount of the solid electrolyte,high mechanical strength and high oxygen ion conductivity can beachieved. The preferred amount of yttria added is 5-8 mol %.

[0087] When Al is further added to a zirconia solid electrolytecontaining yttria in an amount of 0.01-1 weight % based on the totalamount of the solid electrolyte, the sintering temperature of thezirconia solid electrolyte is drastically decreased, thereby improvingthe detection performance of the resultant sensor. Because the additionof Al improves the stability of the electrode interface, the interfaceimpedance can be decreased, resulting in increase in the activity of theelectrode. The preferred amount of Al added is 0.05-0.5 weight %. Whenthe amount of Al added is more than 1 weight %, a solid state reactiontakes place between the solid electrolyte substrate 1 and theelectrode-coating layer 11, resulting in decrease in the sensitivity ofthe gas-detecting element, and decrease in the strength of the zirconiasolid electrolyte. On the other hand, when the amount of Al added isless than 0.01 weight %, there is substantially no effect of adding Al.The thickness of the solid electrolyte substrate 1 is preferably 100-300μm.

[0088] (2) Sensing Electrode

[0089] The sensing electrode 5 should be active with oxygen and adetection object gas. The term “active” used herein means that thesensing electrode 5 generates a predetermined electrode potential whencontacted with oxygen and a detection object gas. This activity may becalled “electrode activity.” The thickness of the sensing electrode 5 ispreferably 2-15 μm.

[0090] The sensing electrode 5 may be formed by a metal oxide and/or aprecious metal (hereinafter referred to as “first precious metal”)active with oxygen and a detection object gas. Though the metal oxideand the first precious metal may be used alone, they are preferably usedin combination to improve the performance of electrodes.

[0091] When the detection object gas is nitrogen oxides (NOx), the useof oxides of Cr as metal oxides provide the sensing electrode 5 withhigh activity. Particularly when at least one selected from the groupconsisting of NiCr₂O₄, FeCr₂O₄, MgCr₂O₄ and Cr₂O₃ is used, it ispossible to provide the sensing electrode 5 with high activity to NOxand high stability. Because these metal oxides are essentially poor insinterability with small sintering shrinkage, strain is likely to begenerated between the resultant sintered body and the solid electrolytesubstrate 1. As described later, solid electrolyte green sheets are usedfor the solid electrolyte substrates 1 in the laminate-type NOx sensorsshown in FIGS. 14-19, resulting in extremely large strain by thesintering shrinkage. Accordingly, particularly when the sensingelectrode 5 is made of a metal oxide only, the lack of theelectrode-coating layer 11 tends to cause cracking and peeling duringthe sintering of the electrodes. In the first embodiment of the presentinvention, by providing an electrode-coating layer 11 made of a solidelectrolyte for functioning to suppress physical strain on a surface ofa sensing electrode 5, the bonding stability of the sensing electrode 5to the solid electrolyte substrate 1 can be improved, thereby improvingthe stability of the sensing electrode 5.

[0092] When the detection object gas is nitrogen oxides (NOx), the useof the first precious metal also provides the sensing electrode 5 withhigh activity. The first precious metal is at least one selected fromthe group consisting of Rh, Ir, Au and precious metal alloys comprisingthese metals. Among the above precious metal alloys, a Pt—Rh alloy is analloy of Pt, which is a precious metal inactive with NOx but active withoxygen (hereinafter referred to as “second precious metal”), and Rhwhich is a first precious metal active with both of NOx and oxygen,exhibiting high sensitivity to NOx and high sensitivity stability.

[0093] The sensing electrode 5 preferably further contains anoxygen-ion-conductive solid electrolyte. Because this increases anelectrode interface and decreases electrode impedance, more stablesensor output can be obtained. The solid electrolytes added to thesensing electrode 5 are preferably the same zirconia solid electrolytesused as the solid electrolyte substrate 1, the electrode-coating layer11 and the electrode underlayer 31 described later. It is preferable toadd at least one selected from the group consisting of magnesia (MgO),ceria (CeO₂), scandia (Sc₂O₃) and yttria (Y₂O₃) as a stabilizer to thezirconia solid electrolyte. The stabilizer is preferably the same as thestabilizer contained in a zirconia solid electrolyte constituting any ofthe solid electrolyte substrate 1, the electrode-coating layer 11 andthe electrode underlayer 31, more preferably the same as the stabilizercontained in zirconia solid electrolyte used for the electrode-coatinglayer 11.

[0094] The amount of the solid electrolyte added is preferably 5-25% bymass, more preferably 10-20% by mass, based on the total amount of thesensing electrode 5. When the amount of the solid electrolyte added isless than 5% by mass, there is no sufficient effect of adding the solidelectrolyte. On the other hand, when the amount of the solid electrolyteadded exceeds 25% by mass, the sensitivity of the gas-detecting elementdecreases.

[0095] (3) Electrode-coating Layer

[0096] The electrode-coating layer 11 covering the sensing electrode 5is bonded to the solid electrolyte substrate 1 directly or via anelectrode underlayer of an oxygen-ion-conductive solid electrolyte. Theelectrode-coating layer 11 should be in such a form that a detection gascan reach a three-phase interface of the solid electrolyte substrate 1,the electrode underlayer or the electrode-coating layer 11 (allconstituted by an oxygen-ion-conductive solid electrolyte) and thesensing electrode 5. In order that the detection object gas can reachthe three-phase interface, the electrode-coating layer 11 is preferablyporous. In addition, the electrode-coating layer 11 covering the sensingelectrode 5 may be a dense layer in such a form that part of sidesurface of the sensing electrode 5 is exposed.

[0097] The porosity of the electrode-coating layer 11 is preferably10-50%. When the porosity is less than 10%, it takes too much time forthe detection object gas to reach the sensing electrode 5, resulting inan elongated gas response time. On the other hand, when the porosity ismore than 50%, the electrode-coating layer 11 has low strength, failingto mechanically suppress strain between the sensing electrode 5 and thesolid electrolyte substrate 1. As a result, the interface between themtends to become unstable by thermal stress caused by the difference in athermal expansion coefficient between the sensing electrode 5 and thesolid electrolyte substrate 1 at the time of detecting a gas, failing toobtain a stable detection output. The more preferred porosity of theelectrode-coating layer 11 is 25-50%.

[0098] The thickness of the electrode-coating layer 11 is important toimprove bonding stability between the solid electrolyte substrate 1 andthe sensing electrode 5. When the electrode-coating layer 11 isconstituted by a porous solid electrolyte, its average thickness ispreferably 3-20 μm. When the thickness of the electrode-coating layer 11is less than 3 μm, the electrode-coating layer 11 per se has too lowstrength. On the other hand, when the thickness of the electrode-coatinglayer 11 is more than 20 μm, it takes too much time for the detectionobject gas to diffuse to the sensing electrode 5, resulting in anelongated gas response time.

[0099] It is preferable to use an oxygen-ion-conductive zirconia solidelectrolyte for the electrode-coating layer 11 from the aspect ofstability or cost reduction. This zirconia solid electrolyte contains atleast one selected from the group consisting of yttria (Y₂O₃), ceria(CeO₂), magnesia (MgO) and scandia (Sc₂O₃) as a stabilizer from theaspect of sensor performance.

[0100] The amount of the stabilizer added is preferably 3-20 mol %, morepreferably 5-20 mol %, particularly 5-15 mol %, based on the totalamount of the solid electrolyte. When the amount of the stabilizer addedis less than 3 mol %, the solid electrolyte has insufficient oxygen ionconductivity, resulting in a decreased sensor output. On the other hand,when the amount of the stabilizer added exceeds 20 mol %, the strengthof the electrode-coating layer 11 decreases, resulting in decrease instability and increase in the fluctuation of output. The stabilizeradded is preferably uniformly dispersed in zirconia and completelydissolved in a solid phase thereof, though a trace amount of thestabilizer may microscopically remain in grain boundaries, etc. withoutaffecting the effects of the present invention.

[0101] The first precious metal active with a detection object gas andoxygen may be added to the electrode-coating layer 11. For instance,when NOx is a detection object, it is preferable to add as the firstprecious metal at least one selected from the group consisting of Au, Irand Rh to the electrode-coating layer 11. This improves the physical andchemical bonding of the sensing electrode 5 to the solid electrolytesubstrate 1, so that resistance in the electrode interface can bereduced. Further, the stability of the electrode interface is soimproved that the drift of sensor output can drastically be reduced. Theamount of the first precious metal added to the electrode-coating layer11 is preferably 0.1-30% by mass, more preferably 1-20% by mass, basedon the total amount of the electrode-coating layer 11, to achieve stableelectrode performance. When the amount of the first precious metal addedis less than 0.1% by mass, its addition effect cannot sufficiently beobtained. On the other hand, when the amount of the first precious metaladded is more than 30% by mass, the ion conductivity of theelectrode-coating layer 11 decreases.

[0102] The second precious metal inactive with a detection object gasand active with oxygen may be added to the electrode-coating layer 11.When the detection object is, for instance, NOx, the second preciousmetal is preferably at least one selected from the group consisting ofPt, Pd and Ru to obtain a stable electrode performance. When the secondprecious metal alone is added to the electrode-coating layer 11, itsamount is preferably 0.05-4% by mass, more preferably 0.1-2% by mass.When the amount of the second precious metal added is less than 0.05% bymass, there is no sufficient effect of adding the second precious metal.On the other hand, when the amount of the second precious metal addedexceeds 4% by mass, the sensor output decreases.

[0103] Both the first and second precious metals can be added to theelectrode-coating layer 11. In this case, the amount of the firstprecious metal added is preferably 0.1-20% by mass, more preferably1-15% by mass, and the amount of the second precious metal added ispreferably 0.05-4% by mass, more preferably 0.1-2% by mass.

[0104] (4) Reference Electrode

[0105] The reference electrode 7 is opposing the sensing electrode 5 viathe solid electrolyte substrate 1 in a reference atmosphere (air)separated from the detection gas. Such structure is necessary to isolatethe reference electrode 7 from the detection gas atmosphere, when thereference electrode 7 has activity with the detection object gas (forinstance, NOx). When the reference electrode 7 is active with thedetection object gas, the solid electrolyte substrate 1 should be madeof a material through which the detection object gas cannot be diffused.On the other hand, when the reference electrode 7 is made of a materialinactive with a detection object gas, the reference electrode 7 may beexposed to a detection gas atmosphere. In this case, other structuresthan shown in FIG. 1 are possible: the sensing electrode 5 and thereference electrode 7 may be disposed on the same surface of the solidelectrolyte substrate 1 (see FIGS. 6-8). The reference electrode 7should be active with at least oxygen and preferably has the same oxygenactivity as that of the sensing electrode 5. When the referenceelectrode 7 is inactive with the detection object gas, the solidelectrolyte substrate 1 may be porous so that the detection object gascan be diffused.

[0106] In the case of detecting NOx, the constituent material for thereference electrode 7 inactive with the detection object gas and activewith oxygen is preferably Pt. The term “inactive with” used herein meansthat the potential of an electrode is sufficiently lower than that ofthe sensing electrode 5 in the same concentration of a detection objectgas (for instance, NOx). The thickness of the reference electrode 7 ispreferably 3-10 μm. Incidentally, though the electrode-coating layer 11is formed only on the sensing electrode 5 in FIG. 1, theelectrode-coating layer 11 may also be formed on the reference electrode7 described later.

[0107] (5) Other Constituents

[0108] The sensing electrode 5 and the reference electrode 7 areconnected via lead conductors provided with a potentiometer 25, wherebypotential difference between the sensing electrode 5 and the referenceelectrode 7 can be measured. The potentiometer 25 may be a usualvoltmeter (circuit). Because current is taken out to a measurementsystem by the voltmeter 25, the voltmeter 25 preferably has sufficientlylarger input impedance than electrode impedance to achieve precisesensor output.

[0109] There is a method without using a voltmeter to measure potentialdifference between the sensing electrode 5 and the reference electrode7. For instance, the detection element is connected to comparison cells(battery) in parallel, to measure the voltage of the comparison cells,at which no current flows between the two cells. In this method, it ispossible to measure the electromotive force of a sensor even though nocurrent is taken out from the detection element at all. Both the sensingelectrode 5 and the reference electrode 7 are preferably provided withelectric current collectors (conductor leads, not shown) made of Pt,etc. The collector may be formed on either a bottom or upper surface ofthe sensing electrode 5 and the reference electrode 7.

[0110] To achieve high ion conductivity, at least the sensing electrode5 and the solid electrolyte 11 are preferably heated at a predeterminedtemperature. Specifically, in the case of zirconia solid electrolyte,they are preferably held at a temperature of 300-400° C. or higher, atwhich ion conductivity increases. A means for heating the sensingelectrode 5 and the solid electrolyte 11 may be an external heat sourceor a self-heating-type heater integrated into the gas-detecting element.

[0111] (B) Modifications of Gas-detecting Element with Coated SensingElectrode

[0112]FIG. 2 is a schematic cross-sectional view showing another exampleof the gas-detecting element of the present invention. In FIG. 2, thesame reference numerals are assigned to parts operating substantially inthe same manner as in FIG. 1. In the structure shown in FIG. 2, asensing electrode 5 is fixed in a recess 1 a formed in a solidelectrolyte substrate 1, and the sensing electrode 5 is covered by anelectrode-coating layer 11′ having gas-diffusing pores 14 such that adetection object gas can be diffused to the sensing electrode 5. Theelectrode-coating layer 11′ is directly bonded to the solid electrolytesubstrate 1 in a region other than the sensing electrode 5. Eachgas-diffusing pore 14 of the electrode-coating layer 11′ has a diameterof preferably 10-1000 82 m, more preferably 100-500 μm.

[0113] A ratio (Sh/Se) of the total opening area (Sh) of thegas-diffusing pores to the area Se of the sensing electrode 5 ispreferably 0.05-0.28, more preferably 0.12-0.28. The thickness of theelectrode-coating layer 11′ having such gas-diffusing pores 14 is 5-100μm, more preferably 30-70 μm. When the thickness of theelectrode-coating layer 11′ is less than 5 μm, there is insufficientcoating effect. On the other hand, when the thickness of theelectrode-coating layer 11′ exceeds 100 μm, there is long diffusion timefor the detection object gas to reach the sensing electrode 5, resultingin slow detection response. Incidentally, because the electrode-coatinglayer 11′ having gas-diffusing pores 14 has sufficient gas diffusioncharacteristics, it need not be porous.

[0114] Of course, in the detection element having a structure in whichthe sensing electrode 5 is fixed in the recess 1 a as shown in FIG. 2,too, the gas-diffusing pores 14 may not be formed if theelectrode-coating layer 11′ is porous. The electrode-coating layer 11′having gas-diffusing pores 14 may be made of the same material asdescribed above with respect to the electrode-coating layer 11 shown inFIG. 1.

[0115]FIG. 3 is a schematic cross-sectional view showing another exampleof the gas-detecting element. In FIG. 3, the same reference numerals areassigned to parts operating substantially in the same manner as inFIG. 1. In this embodiment, an electrode underlayer 31 made of anoxygen-ion-conductive solid electrolyte is formed between the sensingelectrode 5 and the solid electrolyte substrate 1. The electrodeunderlayer 31 is preferably made of the same oxygen-ion-conductive solidelectrolyte as that of the electrode-coating layer 11, and porous likethe electrode-coating layer 11 from the aspect of gas response.

[0116] As shown in FIG. 3, the electrode underlayer 31 may be formedbetween the electrode-coating layer 11 and the solid electrolytesubstrate 1. In this case, though the electrode-coating layer 11 is notdirectly bonded to the solid electrolyte substrate 1, it is bonded tothe solid electrolyte substrate 1 via the solid electrolyte electrodeunderlayer 31 electrically connected thereto. Accordingly, substantiallythe same effect can be obtained as in a case where it is directly bondedto the solid electrolyte substrate 1. In such a structure, even if thesensing electrode 5 is made of a material that is likely to generatestrain when the sensing electrode 5 is directly bonded to the solidelectrolyte substrate 1, stable electrode interface can be formed. Inaddition, a synergistic effect with the strain-suppressing function ofthe electrode-coating layer 11 provides the electrode interface withfurther excellent stability. The electrode underlayer 31 is a dense orporous layer having a thickness of about 3 to 10 μm.

[0117]FIG. 4 is a schematic cross-sectional view showing a still furtherexample of the gas-detecting element. In FIG. 4, the same referencenumerals are assigned to parts operating substantially in the samemanner as in FIG. 1. An electric insulating layer 32 is formed betweenthe sensing electrode 5 and the solid electrolyte substrate 1. Theelectrode-coating layer 11 laminated on a surface of the sensingelectrode 5 has a portion bonded directly to the solid electrolytesubstrate 1. With the electric insulating layer 32, an electrodereaction at the sensing electrode 5 predominantly takes place in bondinginterface with the electrode-coating layer 11. Accordingly, a detectionobject gas can be detected immediately after it reaches a surface of thesensing electrode 5, resulting in excellent gas response.

[0118]FIG. 5 shows a gas-detecting element having basically the samestructure as in FIG. 1, though it is not that an entire surface of asensing electrode 5 is covered by an electrode-coating layer 11 as shownin FIG. 1, but that it has a portion not covered by theelectrode-coating layer 11 in part of its side surface. In thisstructure, because a detection gas can be diffused from a side surfaceof the electrode to a detection part thereof, the electrode-coatinglayer 11 need not be porous but be a dense layer. The term “dense layer”used herein a layer having porosity of 0.5% or less. By covering asurface of a sensing electrode 5 with a dense electrode-coating layer11, the effect of isolating the sensing electrode 5 from foreigncontamination components is remarkably improved. To exhibit the aboveeffect sufficiently, the dense electrode-coating layer 11 preferably hasan average thickness of 1-10 μm.

[0119] FIGS. 6-8 are schematic cross-sectional views showing stillfurther examples of the gas-detecting elements. In FIGS. 6-8, the samereference numerals are assigned to parts operating substantially in thesame manner as in FIG. 1. Each of the gas-detecting elements shown inFIGS. 6-8 has a structure in which the sensing electrode 5 and thereference electrode 7 are disposed on the same surface of the solidelectrolyte substrate 1. In such structure, because the sensingelectrode 5 and the reference electrode 7 are exposed to a detection gasatmosphere simultaneously, the reference electrode 7 should be inactivewith at least a detection object gas. In the gas-detecting element shownin FIG. 7, electrode-coating layers 11, 12 are formed on the sensingelectrode 5 and the reference electrode 7, respectively. The function ofthe electrode-coating layer 12 on the reference electrode 7 will bedescribed later referring to FIG. 11.

[0120] When the sensing electrode 5 and the reference electrode 7 aredisposed on the same surface of the solid electrolyte substrate 1, asshown in FIG. 8, an electric insulating substrate 41 is laminated withthe solid electrolyte substrate film layer 1′ and then with the sensingelectrode 5 and the reference electrode 7.

[0121]FIGS. 9 and 10 are schematic cross-sectional views showing stillfurther examples of the gas-detecting elements. In FIGS. 9 and 10, thesame reference numerals are assigned to parts operating substantially inthe same manner as in FIG. 1. FIGS. 9 and 10 show structures in whichtwo sensing electrodes 5 a, 5 b (hereinafter referred to as “firstsensing electrode” and “second sensing electrode,” respectively) aredisposed via an electrode-coating layer 11.

[0122] In the structure shown in FIG. 9, the first sensing electrode 5 ais directly fixed onto the solid electrolyte substrate 1 and covered bythe electrode-coating layer 11, and the second sensing electrode 5 b isfixed onto the electrode-coating layer 11. In the structure shown inFIG. 10, the first sensing electrode 5 a is directly fixed onto thesolid electrolyte substrate 1 and covered by the electrode-coating layer11, and the second sensing electrode 5 b is embedded in theelectrode-coating layer 11 at a position above the first sensingelectrode 5 a. Though the first sensing electrode 5 a and the secondsensing electrode 5 b are separated from each other with a certain gapin the illustrated example, they may be in partial contact with eachother. Thus, a sensing electrode composed of the sensing electrodes 5 aand 5 b has a wider electrode interface area, resulting in decreasedelectrode impedance.

[0123] The gas-detecting element having the structure shown in FIG. 10exhibits excellent gas response, because an electrode reaction on thefirst and second sensing electrodes 5 a, 5 b takes place predominantlyin a bonding interface with the electrode-coating layer 11. Of course,in addition to the structures shown in FIGS. 9 and 10, for instance,three or more sensing electrodes may be formed, and the arrangement ofthese electrodes is not particularly restrictive as long as it is insideor on the electrode-coating layer 11. Incidentally, any of the firstsensing electrode 5 a and the second sensing electrode 5 b may be formedby the same material as that of the above sensing electrode 5. This istrue when three or more sensing electrodes are formed.

[0124] (C) One Example of Gas-detecting Element with Coated ReferenceElectrode

[0125] The basic structure of the gas-detecting element of the presentinvention in which a reference electrode is covered by anelectrode-coating layer will be explained. FIG. 11 is a schematiccross-sectional view showing one example of a gas-detecting element witha coated reference electrode. The structure of this gas-detectingelement is basically the same as shown in FIG. 1, except that thereference electrode 7, in place of the sensing electrode 5, is coveredby an electrode-coating layer 12, and that the reference electrode 7 isalso exposed to a detection gas atmosphere. Formed on anoxygen-ion-conductive solid electrolyte substrate 1 are a sensingelectrode 5 active with a detection object gas and oxygen on one surfaceand a reference electrode 7 on the other surface.

[0126] The porous, oxygen-ion-conductive, electrode-coating layer 12 islaminated on the reference electrode 7, such that the electrode-coatinglayer 12 is directly in contact with the solid electrolyte substrate 1.By direct contact with the solid electrolyte substrate 1, theelectrode-coating layer 12 also functions as the solid electrolytesubstrate 1 of the reference electrode 7, whereby the electrodeinterface impedance of the reference electrode 7 can be reduced.Accordingly, the electrode reaction speed increases, resulting inimprovement in stability.

[0127] Though the electrode-coating layer 12 is directly bonded to thesolid electrolyte substrate 1 in the example of FIG. 11, the same effectcan also be obtained by bonding via the electrode underlayer 31 made ofan oxygen-ion-conductive solid electrolyte as in the electrode-coatinglayer 11 shown in FIG. 3. Though the sensing electrode 5 is not coveredby the electrode-coating layer 12 in the structure shown in FIG. 11, ofcourse, both of the sensing electrode 5 and the reference electrode 7may be covered by the electrode-coating layer 12. The sensing electrode5 and the reference electrode 7 are connected via lead conductorsprovided with a potentiometer 25, thereby making it possible to measurepotential difference between the sensing electrode 5 and the referenceelectrode 7.

[0128] (D) Modifications of Gas-detecting Element with Coated ReferenceElectrode

[0129]FIG. 12 shows a gas-detecting element having basically the samestructure as shown in FIG. 11, except that it is not that an entiresurface of the reference electrode 7 is covered by an electrode-coatinglayer 12 like in FIG. 11, but that part of a side surface of theelectrode has a portion not covered by the electrode-coating layer 12.With this structure, a detection gas can be diffused from a side surfaceof the electrode to its detection part. Thus, it is not necessary tomake the electrode-coating layer 12 porous, but it may be a dense layer.Thus, by covering a surface of the reference electrode 7 with a denseelectrode-coating layer 12, the effect of preventing foreigncontamination components from coming into contact with the referenceelectrode 7 is remarkably improved. As a result, it becomes easier toconstitute a reference electrode active only with oxygen withoutgenerating activity with a detection object gas.

[0130] Further, because the electrode-coating layer 12 has a portionwith which it is brought into direct contact with the solid electrolytesubstrate 1, impedance reduction effect can also be obtained asdescribed above. Though the side surface of the electrode acts as a gasinlet in FIG. 12, the same effect can be obtained, for instance, byproviding the electrode-coating layer 12 with a diffusion opening assmall as a pinhole on an upper surface of the electrode like the coatinglayer 11′ on the sensing electrode shown in FIG. 2. In this case,however, it is necessary that the cross section area, number, etc. ofthe pinhole should be designed taking contamination prevention effectand response performance into consideration.

[0131] As described above, the electrode-coating layer 12 covering thereference electrode 7 exposed to a detection gas should be in a form inwhich a detection gas can reach a three-phase interface with thereference electrode 7 and the solid electrolyte substrate 1. In thiscase, if the electrode-coating layer 12 is provided with diffusion pores14, or part of a side surface of the reference electrode 7 is exposed,it is not necessary that the electrode-coating layer 12 is constitutedby a porous material. Particularly to prevent contamination duringsintering or operation, it is preferable to cover an upper surface ofthe reference electrode 7 with a dense layer, such that part of a sidesurface of the electrode is exposed. To reduce interface impedancebetween the reference electrode 7 and the solid electrolyte substrate 1,at least part of the electrode-coating layer 12 should be bonded to thesolid electrolyte substrate 1 directly or via the electrode underlayer31 made of an oxygen-ion-conductive solid electrolyte.

[0132] The porosity of the electrode-coating layer 12 is preferably0-50%, though it is changeable depending on its structure. When theelectrode-coating layer 12 is porous, the electrode-coating layer 12preferably has an average thickness of 1-20 μm to provide thegas-detecting element with good performance. On the other hand, when theelectrode-coating layer 12 is a dense layer, its average thickness ispreferably 1-10 μm to obtain sufficient effect.

[0133] The electrode-coating layer 12 covering the reference electrode 7is preferably made of the same oxygen-ion-conductive zirconia solidelectrolyte as that of the electrode-coating layer 11 covering thesensing electrode 5. The electrode-coating layer 12 may contain thesecond precious metal inactive with a detection object gas and activewith oxygen. For instance, when the detection object is NOx, the secondprecious metal is preferably at least one selected from the groupconsisting of Pt, Pd and Ru to provide stable electrode performance. Theamount of the second precious metal added is preferably 0.05-4% by mass,more preferably 0.1-2% by mass, based on the total amount of theelectrode-coating layer 12. When the amount of the second precious metaladded is less than 0.05% by mass, there is no sufficient effect ofadding the second precious metal. On the other hand, when the amount ofthe second precious metal added exceeds 4% by mass, there issubstantially no further improvement in the effect, resulting only inincrease in the product cost.

[0134] The reference electrode 7 need only be made of an electrodematerial active with oxygen. Particularly in a structure in which thereference electrode 7 is also disposed in a detection gas atmosphere, itis preferably made of an electrode material inactive with a detectionobject gas and active only with oxygen. When the detection object gas isNOx, a material comprising at least one selected from the groupconsisting of platinum, iridium and gold is preferable because of itsrelatively low electrode potential to NOx. Particularly the referenceelectrode made of platinum and iridium has low electrode potential toNOx, thereby making it possible to reduce the impedance of the electrodeper se.

[0135] The reference electrode also preferably contains anoxygen-ion-conductive solid electrolyte. The solid electrolyte added tothe reference electrode is preferably a zirconia solid electrolyte. Inthis case, it is more possible to add as a stabilizer at least oneselected from the group consisting of magnesia, ceria, scandia andyttria. When the stabilizer added to a solid electrolyte for theelectrode-coating layer 12 is the same as added to a solid electrolytefor the reference electrode 7, better effect can be obtained.

[0136] Though the structure in which the reference electrode 7 and thesensing electrode 5 are exposed to the same detection gas has beenexplained above, the reference electrode 7 can also be covered by theelectrode-coating layer 12 in the structure in which it is exposed to areference gas such as air, etc. In this case, the prevention ofcontamination need not be considered, but improvement in the bondingstability of interface between the solid electrolyte substrate 1 and thereference electrode 7 and gas response need only be considered in theconstruction of the electrode-coating layer 12.

[0137] (E) Gas-detecting Element with Coated Oxygen-sensing Electrode

[0138]FIG. 13 shows the structure of the gas-detecting element of thepresent invention, in which an oxygen-sensing electrode is covered by anelectrode-coating layer. In this gas-detecting element, a sensingelectrode 5 and an oxygen-sensing electrode 6 are disposed on the samesurface of the solid electrolyte substrate 1, and a reference electrode7 is disposed on the opposite surface of the solid electrolyte substrate1 such that it is opposing the sensing electrode 5 and theoxygen-sensing electrode 6 via the solid electrolyte substrate 1. Thereference electrode 7 is in a reference gas atmosphere separated fromthe detection object gas. The oxygen-sensing electrode 6 is covered byan electrode-coating layer 13 made of an oxygen-ion-conductive solidelectrolyte. Such structure is effective when the oxygen-sensingelectrode 6 has activity with a trace amount of the detection objectgas. In this case, by isolating the reference electrode 7 from adetection gas atmosphere, the concentration of a detection object gascan be detected with high precision.

[0139] The solid electrolyte substrate 1 should be resistant to gasdiffusion. The oxygen-sensing electrode 6 is preferably active at leastwith oxygen. The electrode-coating layer 13 covering the oxygen-sensingelectrode 6 is preferably the same as the electrode-coating layer 12covering the reference electrode 7 in shape, size, material, etc.

[0140] The oxygen-sensing electrode 6 need only be made of an electrodematerial active with oxygen, but it is preferably made of an electrodematerial inactive with a detection object gas and active only withoxygen. When the detection object gas is NOx, a material comprising atleast one selected from the group consisting of platinum, iridium andgold is preferable because of its relatively low electrode potential toNOx. Particularly in the case of the oxygen-sensing electrode made of aplatinum-iridium alloy having low electrode potential to NOx, theimpedance of the electrode per se can be lowered.

[0141] [2] Production Method of Gas-detecting Element

[0142] Though there is no limitation in the production method of thegas-detecting element, it will be explained taking as an example where azirconia green sheet is used. The use of a zirconia green sheet provideshigh productivity to the gas-detecting element. Zirconia powder as astarting material is preferably zirconia powder containing apredetermined amount of Y₂O₃, though zirconia powder and yttria powdermay be mixed at a predetermined ratio. The starting material power ismixed with predetermined amounts of a binder and a solvent, blended by aball mill, etc., and formed into a sheet by a doctor blade method, aninjection method, etc.

[0143] When the gas-detecting elements shown in FIGS. 1-13 are produced,an electrode paste is applied onto a zirconia green sheet or a sinteredsolid electrolyte substrate by a screen-printing method, etc., to form asensing electrode 5 and a reference electrode 7, and further anoxygen-sensing electrode 6 if necessary. If necessary, after repeatingdrying and printing, lead conductors and electrode-coating layers aresimilarly screen-printed. After the completion of screen-printing, thegreen sheet is degreased at about 500° C., and then sintered usually at1400° C. or higher. Finally, lead wires are welded to collectorterminals made of Pt, etc.

[0144] [3] Gas-detecting Device

[0145] The gas-detecting elements (detection cells) having the basicstructures shown in FIGS. 1-13 can detect nitrogen oxides, hydrocarbon,carbon monoxide, ammonia, etc., exhibiting excellent effect particularlyin the measurement of nitrogen oxides. Thus, detailed explanation willbe given below with respect to a case where the gas-detecting device,into which the gas-detecting element of the present invention isassembled, is used for the detection of nitrogen oxides. Of course, thegas-detecting device of the present invention is effective to otherdetection object gases than nitrogen oxides, too.

[0146] (A) First Gas-detecting Device

[0147]FIG. 14 is a schematic cross-sectional view showing one example ofthe first gas-detecting device for measuring nitrogen oxides. Thisgas-detecting device is a laminate-type NOx sensor, which comprises (a)a gas-measuring chamber 4 defined by first and secondoxygen-ion-conductive solid electrolyte substrates 1 and 2 disposed witha predetermined gap; (b) a gas inlet 3 provided so that a detection gasflows into the gas-measuring chamber 4 with a predetermined gasdiffusion resistance; (c) a NOx-detecting cell comprising a sensingelectrode 5 (hereinafter referred to as “NOx-sensing electrode”) fixedonto the first solid electrolyte substrate 1 such that it is exposed toan atmosphere in the gas-measuring chamber 4, and active with NOx andoxygen, and a reference electrode 7 fixed onto the first solidelectrolyte substrate 1 and active with oxygen; (d) a NOx-convertingpump element comprising (i) a NOx-converting electrode 8 fixed onto thesecond solid electrolyte substrate 2 such that it is exposed to anatmosphere in the gas-measuring chamber 4, and active with NOx andoxygen, and (ii) a NOx-converting counter electrode 9 fixed onto thesecond solid electrolyte substrate 2 such that it is exposed to anatmosphere in a duct 16 containing oxygen and/or an oxide gas, andactive with oxygen, which can convert NO to NO₂, or NO₂ to NO in thedetection gas.

[0148] This gas-detecting device further comprises (e) a means 25 formeasuring the potential difference between the NOx-sensing electrode 5and the reference electrode 7, and (f) a voltage-applying means 28 fordriving the NOx-converting pump element, whereby the concentration ofNOx in the detection gas can be determined by detecting the potentialdifference between the NOx-sensing electrode 5 and the referenceelectrode 7 while applying predetermined voltage to the NOx-convertingpump element. At least the NOx-sensing electrode 5 in the NOx-detectingcell is covered by the electrode-coating layer 11 made of anoxygen-ion-conductive solid electrolyte in such a form as to make itpossible for a detection gas to reach an interface between theNOx-sensing electrode 5 and the first solid electrolyte substrate 1, theelectrode underlayer or the electrode-coating layer. Theelectrode-coating layer 11 has a portion bonded to the first solidelectrolyte substrate 1 directly or via an electrode underlayer (notshown) made of an oxygen-ion-conductive solid electrolyte.

[0149] The gas-detecting device is provided with a heater 18 (forinstance, self-heating-type heater) for heating the NOx-detecting cellcomprising the NOx-sensing electrode 5, the reference electrode 7 andthe electrode-coating layer 11 to a predetermined temperature, and theheater 18 is integrally sandwiched by heater substrates 19 a and 19 b.

[0150]15 a and 22 respectively indicate spacers for holding the firstsolid electrolyte substrate 1 and the second solid electrolyte substrate2 at a predetermined gap, and the spacer 22 has a gas inlet 3. Also, 15b indicates a spacer for providing an air duct 16 communicating with theNOx-converting counter electrode 9, 15 c indicates a spacer forproviding an air duct 17 communicating with the reference electrode 7.Further, 23 indicates a substrate for defining the air duct 16.

[0151] In the gas-detecting device shown in FIG. 14, theelectrode-coating layer 11′ provided with gas-diffusing pores as shownin FIG. 2 may be used in place of the porous electrode-coating layer 11.The solid electrolyte substrates 1 and 2 are preferably made of the samezirconia solid electrolyte as above, and the spacer 15 a is alsopreferably made of the same zirconia solid electrolyte. The heatersubstrates 19 a and 19 b sandwiching the heater 18 is preferably made ofa zirconia solid electrolyte. In this case, an alumina layer, etc.having high electric insulation are preferably disposed in eachinterface between the heater substrates 19 a and 19 b and the heater 18.

[0152] Because the laminate-type NOx sensor shown in FIG. 14 has anelectrochemical oxygen pump (NOx-converting pump element comprisingNOx-converting electrode 8 and conversion counter electrode 9), it ispossible to convert NO to NO₂ in a combustion exhaust gas to provide adetection gas, in which NOx is constituted by NO₂ only, or to convertNO₂ to NO in a combustion exhaust gas to provide a detection gas, inwhich NOx is constituted by NO only, depending on conditions, therebydetecting the total NOx concentration in the detection gas.

[0153] The conversion of a gas comprising a plurality of detectionobject gases to a gas comprising only one detection object gas by usingsuch NOx-converting pump element can be carried out by introducingoxygen into the gas-measuring chamber 4 from outside to oxidize NO bythe NOx-converting electrode 8, or by discharging oxygen from thegas-measuring chamber 4 to reduce NO₂ by the NOx-converting electrode 8.The structure shown in FIG. 14 is an example in which the conversioncounter electrode 9 is disposed in the air duct 16 with oxygen pumpedfrom air. However, the conversion counter electrode 9 can be disposed inthe gas-measuring chamber 4, so that the conversion counter electrode 9is exposed to a detection gas atmosphere, to electrochemically decomposean oxide in the detection gas for oxygen pumping. Oxides in thedetection gas are usually CO₂, CO, H₂O, etc.

[0154] The material forming the conversion electrode 8 is preferably atleast one precious metal selected from the group consisting of platinum,rhodium, iridium, gold and alloys containing these metals. The alloysmay be a Pt—Rh alloy, an Ir—Rh alloy, a Pt—Ru alloy, etc. Particularlywhen the conversion electrode formed by a platinum-rhodium alloy such asPt-5.5 mol %Rh, etc. is used, good NOx conversion can be carried out. Inaddition, when the conversion electrode 8 is formed by a mixture of atleast one precious metal selected from the group consisting of platinum,rhodium, iridium, gold and alloys containing these metals and at leastone metal oxide selected from the group consisting of Cr₂O₃, NiO,NiCr₂O₄, MgCr₂O₄ and FeCr₂O₄, the electrode 8 can be provided withexcellent conversion stability. Incidentally, the material forming theconversion counter electrode 9 is not particularly restrictive as longas it is active with oxygen, though it is preferably Pt, Pd, Ir, etc.,particularly preferably Pt.

[0155] With a structure having the electrode-coating layer 11 formed onthe NOx-sensing electrode 5, it is possible to solve the problems of theinstability of electrode interface, output drift, etc. due to thermalstrain of the electrode, thereby stabilizing the electrochemicalactivity of electrode interface. In addition, because the interfaceimpedance of the electrode can be reduced, it is possible to improveresponse performance at the time of detecting a gas. Because thereference electrode 7 active with oxygen is disposed in the air duct 17in this sensor structure, it is completely isolated from a detectiongas, so that it can function as a reference electrode. The material ofthe reference electrode 7 is usually Pt, it is possible to add an oxygenion conductor such as a zirconia solid electrolyte to improve oxygenactivity.

[0156] Though, in the example of FIG. 14, only the NOx-sensing electrode5 is covered by the electrode-coating layer 11, the reference electrode7 disposed in the air duct 17, of course, may also be covered by theelectrode-coating layer 12. This improves the electrode interfacestability of not only the NOx-sensing electrode 5 but also the referenceelectrode 7, resulting in decrease in interface impedance. As a result,the change ratio of drift further decreases in the gas-detecting device,resulting in improvement in gas detection stability.

[0157]FIG. 15 is a schematic cross-sectional view showing anotherexample of the gas-detecting device, and FIG. 16 is its plan view. FIG.16 shows the in-plane arrangement of each part of the gas-detectingdevice. Incidentally, in other examples of the gas-detecting devices,too, the in-plane arrangement of each element is substantially the sameas in FIG. 16. In FIG. 15, the same reference numerals are assigned toparts operating substantially in the same manner as in FIG. 14. In thegas-detecting device shown in FIG. 15, the reference electrode 7 isdisposed on the same surface as the sensing electrode 5, thereby beingexposed to a detection gas atmosphere. The reference electrode 7 shouldbe active with oxygen but inactive with NOx. The material forming suchreference electrode 7 is preferably Pt.

[0158] When the reference electrode 7 active with oxygen but inactivewith NOx is used in such a mixed-potential-type sensor, the referenceelectrode 7 can be disposed in the same detection gas atmosphere as theNOx-sensing electrode 5. The reference electrode 7 is preferablydisposed in the vicinity of the NOx-sensing electrode 5. Though both ofthe NOx-sensing electrode 5 and the reference electrode 7 are covered bythe electrode-coating layers 11, 12 in FIG. 15, the covering of eitherone of them, of course, provides a considerable effect. Theelectrode-coating layers 11, 12 act to improve the interface bondingstability of the solid electrolyte substrate/the electrode and gasresponse.

[0159] In such a structure that the reference electrode 7 is opposingthe NOx-converting electrode 8, the covering of the reference electrode7 with the electrode-coating layer 12 provides remarkable effect ofpreventing contamination. Namely, the electrode-coating layer 12 canefficiently prevent the phenomenon that contamination componentsgenerated from the NOx-converting electrode 8 at the time of sinteringor using the gas-detecting device provide the reference electrode withactivity with NOx.

[0160]FIG. 17 is a schematic cross-sectional view showing a furtherexample of the gas-detecting device. In FIG. 17, the same referencenumerals are assigned to parts operating substantially in the samemanner as in FIGS. 14 and 15. FIG. 17 shows a structure in which anoxygen sensing electrode 6 active with oxygen but inactive with NOx isdisposed in a gas-measuring chamber 4, and a reference electrode 7 forboth of the NOx-sensing electrode 5 and the oxygen sensing electrode 6is disposed in an air duct 17. The material forming the oxygen-sensingelectrode 6 is preferably Pt, Pd, Ir, a Pt—Ir alloy, etc. Of course, tworeference electrodes may be arranged separately for the NOx-sensingelectrode 5 and the oxygen-sensing electrode 6.

[0161] With an arithmetic treatment means 27 using potential differenceE₂ between the reference electrode 7 and the oxygen-sensing electrode 6and potential difference E₁ between the reference electrode 7 and theNOx-sensing electrode 5, corrections are made to the variation of oxygenconcentration. The arithmetic treatment means 27 may be hardware usingan electronic circuit, or software using a microcomputer, etc. Thismakes it possible to detect NOx with high precision even when thevariation of oxygen concentration in a detection gas atmosphereinfluences the oxygen concentration in the gas-measuring chamber 4.

[0162] Though in the example shown in FIG. 17, both of the NOx-sensingelectrode 5 and the oxygen-sensing electrode 6 are covered by theelectrode-coating layers 11, 13, the reference electrode 7, of course,may be covered by the electrode-coating layer 12. In the structure inwhich the oxygen-sensing electrode 6 is opposing the NOx-convertingelectrode 8, the electrode-coating layer 13 is effective to preventcontamination from the NOx-converting electrode 8, whereby theelectrode-coating layer 13 efficiently prevents the phenomenon thatoxygen-sensing electrode 6 is given activity with NOx, like thereference electrode 7 shown in FIG. 15. The preferred example of theelectrode-coating layer 13 is the same as the above electrode-coatinglayer 12 covering the reference electrode 7 opposing the NOx-convertingelectrode 8.

[0163]FIG. 18 is a schematic cross-sectional view showing a stillfurther example of the gas-detecting device. In FIG. 18, the samereference numerals are assigned to parts operating substantially in thesame manner as in FIGS. 14-17. The laminate-type NOx sensor shown inFIG. 18 is constituted by adding, to the structure shown in FIG. 17, agas-treating electrode 10 for oxidizing a reducing gas in a detectiongas atmosphere (for instance, CO, HC, etc. in a combustion exhaust gas)in a front part of the gas-measuring chamber 4. The gas-treatingelectrode 10 is active with HC, CO, etc. Though FIG. 18 shows an examplein which the conversion counter electrode 9 serves as a counterelectrode for both of the gas-treating electrode 10 and theNOx-converting electrode 8, two conversion counter electrodes 9 may bearranged separately for the gas-treating electrode 10 and theNOx-converting electrode 8.

[0164] The gas-detecting device shown in FIG. 18 comprises agas-treating pump element constituted by the gas-treating electrode 10and its counter electrode 9, and an external power supply 29 as a meansfor applying voltage to the gas-treating pump element. With agas-flow-resisting member 24 having a gas-passing aperture 30 disposedbetween the gas-treating electrode 10 and the NOx-converting electrode8, the gas-measuring chamber 4 may be turned into a two-chamberstructure having a gas-converting chamber 4 a (first chamber) and agas-measuring chamber 4 b (second chamber). By increasing oxideconcentration in the gas-converting chamber 4 a, the variation of oxygenconcentration in the gas-measuring chamber 4 b is suppressed, resultingin increase in sensor performance, thereby achieving high-precision NOxdetection. The material forming the gas-treating electrode 10 ispreferably Pt, Pd, Ir, Au, Rh, etc.

[0165]FIG. 19 is a schematic cross-sectional view showing a stillfurther example of the gas-detecting device. In FIG. 19, the samereference numerals are assigned to parts operating substantially in thesame manner as in FIGS. 14-18. In the laminate-type NOx sensor shown inFIG. 19, a plurality of gas-passing apertures 21 are provided such thatthey penetrate a solid electrolyte substrate 1 on which a NOx-sensingelectrode 5 and a solid electrolyte substrate 1 b are formed. A regionof the solid electrolyte substrate 1, in which the gas-passing apertures21 open, is covered by a porous body 20 functioning as a spacer for anair duct 17. The porous body 20 has pores constituting gas inlets 3, inwhich an oxidation catalyst is carried.

[0166] The detection gas enters into the pores of the porous body 20, inwhich HC (hydrocarbons) and CO are oxidized by an oxidation catalyst toincrease oxide concentration. After the detection gas passes through thegas-passing apertures 21, it is diffused into a gas-treating electrode10 constituted by a porous layer sandwiched by the solid electrolytesubstrates 1 b and 2 b, to further increase oxide concentration. Withsuch structure, a gas treatment effect is increased. The porosity of theporous body 20 carrying the oxidation catalyst and the gas-treatingelectrode 10 are designed preferably so that the gas-flowing resistanceis not a parameter determining a gas diffusion rate. The conversioncounter electrode 9 b for the gas-treating electrode 10 is disposedseparately from the conversion counter electrode 9 a for theNOx-converting electrode 8. To increase the uniformity of heatingtemperature of a sensor element, a pair of heaters 18 a and 18 b arearranged in the substrate on both sides of the gas-detecting element.

[0167] (B) Second Gas-detecting Device

[0168]FIG. 21 is a schematic cross-sectional view showing an example ofthe second gas-detecting device (laminate-type NOx sensor). In FIG. 21,the same reference numerals are assigned to parts operatingsubstantially in the same manner as in FIG. 14. This gas-detectingdevice is different from the first gas-detecting device shown in FIG. 14in the structure of a NOx-converting pump element. Accordingly, detailedexplanation will be made below with respect to the NOx-converting pumpelement.

[0169] In the NOx-converting pump element of the second gas-detectingdevice, at least a NOx-converting electrode 8 is covered by anelectrode-coating layer 51 made of an oxygen-ion-conductive solidelectrolyte. The electrode-coating layer 51 is in such a form that adetection gas can reach a three-phase interface of a solid electrolytesubstrate, an electrode underlayer or an electrode-coating layer andeach electrode. The electrode-coating layer 51 has a portion bonded tothe second solid electrolyte substrate 2 directly (FIG. 21) or via anelectrode underlayer 52 made of an oxygen-ion-conductive solidelectrolyte (FIG. 22). Accordingly, the fixing of the NOx-convertingelectrode 8 covered by the electrode-coating layer 51 to the secondsolid electrolyte substrate 2 is reinforced. The electrode-coating layer51 eliminates the bonding instability of interface between the secondsolid electrolyte substrate 2 and the NOx-converting electrode 8 due tothermal stress caused by the difference in a thermal expansioncoefficient therebetween. In addition, the NOx-converting electrode 8covered by the electrode-coating layer 51 is prevented from directcontact with a remaining reducing gas, whereby electrochemical activitycontributing to the function of NOx conversion is stabilized.

[0170] The electrode-coating layer 51 is preferably a porous solidelectrolyte layer, through which a detection gas (detection object gas)is diffusible, and the porosity of the porous solid electrolyte filmlayer is preferably 10-50%. When the porosity is less than 10%, it takestoo much time for the detection object gas to diffuse to the conversionelectrode, resulting in elongated gas response time and thusinsufficient NOx conversion. On the other hand, when the porosity ismore than 50%, the electrode-coating layer 51 has poor strength, failingto mechanically suppress strain generated between the NOx-convertingelectrode 8 and the solid electrolyte substrate 2. As a result, theelectrode interface is likely to become unstable by thermal stress dueto the difference between them in a thermal expansion coefficient,failing to obtain stable detection output.

[0171] The thickness of the electrode-coating layer 51 is an importantfactor for obtaining good effects. The electrode-coating film layer 51preferably has an average thickness of 3-20 μm. When the thickness ofthe electrode-coating layer 51 is less than 3 μm, the electrode-coatinglayer 51 per se has too low strength. On the other hand, when thethickness is more than 20 μm, it takes too much time for the detectionobject gas to diffuse to the conversion electrode 8, resulting inelongated gas response time and thus insufficient NOx conversion.

[0172] The electrode-coating layer 51 is made of anoxygen-ion-conductive zirconia solid electrolyte, which preferablycontains at least one selected from the group consisting of yttria(Y₂O₃), ceria (CeO₂), magnesia (MgO) and scandia (Sc₂O₃) as a stabilizerfrom the aspect of sensor performance. The amount of the stabilizeradded is preferably 3-20 mol % based on the total amount of the solidelectrolyte. When the amount of the stabilizer added is less than 3 mol%, there is insufficient oxygen ion conductivity. On the other hand,when it is more than 20 mol %, the electrode-coating layer 51 has lowstrength, resulting in decrease in stability and increase in thevariations of output. The stabilizer added is preferably uniformlydispersed in zirconia, and completely dissolved in a solid phasethereof. However, even if a trace amount of a stabilizer remainsmicroscopically in grain boundaries, etc., the effects of the presentinvention would not be affected.

[0173] In the case of detecting NOx, the oxygen-ion-conductive zirconiasolid electrolyte for the electrode-coating layer 51 preferably furthercontains (a) at least one precious metal selected from the groupconsisting of platinum, rhodium, iridium, gold and alloys containingthese metals, and/or (b) at least one metal oxide selected from thegroup consisting of Cr₂O₃, NiO, NiCr₂O₄, MgCr₂O₄ and FeCr₂O₄. The amountof the precious metal and/or the metal oxide added is preferably in arange of 1-50% by volume (total amount, when both are contained), basedon 100% by volume of the zirconia solid electrolyte.

[0174]FIG. 22 is a schematic cross-sectional view showing anotherexample of the gas-detecting device. In FIG. 22, the same referencenumerals are assigned to parts operating substantially in the samemanner as in FIG. 21. The gas-detecting device shown in FIG. 22 has astructure in which a NOx-converting electrode 8 is formed on a solidelectrolyte substrate 2 via an electrode underlayer 52 made of anoxygen-ion-conductive solid electrolyte. The electrode-coating layer 51not only covers the NOx-converting electrode 8 but also is bonded to theelectrode underlayer 52.

[0175] The electrode underlayer 52 is preferably made of a zirconiasolid electrolyte containing as a stabilizer at least one selected fromthe group consisting of yttria, ceria, magnesia and scandia. Theelectrode underlayer 52 is preferably made of the same material as thoseof the NOx-converting electrode 8 and the electrode-coating layer 51,which more preferably contains at least one precious metal selected fromthe group consisting of platinum, rhodium, iridium, gold and alloyscontaining these metals, and/or at least one metal oxide selected fromthe group consisting of Cr₂O₃, NiO, NiCr₂O₄, MgCr₂O₄ and FeCr₂O₄, in arange of 0.1-20% by volume (total amount, when both are contained). Theelectrode underlayer 52 is a dense or porous layer preferably having athickness of about 3 to 10 μm.

[0176] [4] Production Method of Gas-detecting Device

[0177] In the production of the laminate-type NOx sensors shown in FIGS.14-22, it is preferable to use green sheets as in the case of thegas-detecting element. For instance, in the case of the production ofthe laminate-type NOx sensor shown in FIG. 18, the NOx-sensing electrode5, the oxygen-sensing electrode 6 and the electrode-coating layers 11,13 are screen-printed on one surface of a green sheet I as shown in FIG.20, and the reference electrode 7 is screen-printed on the other surfaceof the green sheet I, and further necessary lead conductors arescreen-printed to form a detection cell. Also, the NOx-convertingelectrode 8 and the gas-treating electrode 10 are screen-printed on onesurface of a green sheet II, and the NOx-converting counter electrode 9is screen-printed on the other surface of the green sheet II, andfurther necessary lead conductors are screen-printed to form aconversion pump element. Further, by sandwiching the heater 18 and itslead conductors with two green sheets for the solid electrolytesubstrates 19 a, 19 b, a sheet III for the heater portion is produced.

[0178] Green sheets for spacers 15 a, 22 and a gas-flow-resisting member24 are sandwiched by the green sheets I and II, and a green sheet for aspacer 15 c is sandwiched by the green sheets I and III, and finally agreen sheet for the substrate 23 for the air duct is laminated on thegreen sheet II via a green sheet for a spacer 15 b. In this case, aportion of the element turning to an internal space is in advance filledor printed with a thermally removable material such as Theobromine,etc., which is sublimed at a degreasing temperature. The resultantlaminate is pressed while heating, degreased at about 500° C., and thensintered, for instance, at 1400° C. or higher. Lead wires of Pt, etc.are finally welded to the collector terminals of the resultant sinteredbody.

[0179] Though the gas-detecting element and the gas-detecting device ineach Example of the present invention have been explained above indetail with respect to their structures and compositions constitutingtheir parts, etc., these explanations are applicable to thegas-detecting elements and the gas-detecting devices in any Examplesunless otherwise mentioned.

[0180] The present invention will be explained in further detail by wayof the following Examples, without intention of restricting the presentinvention thereto.

REFERENCE EXAMPLE 1

[0181] NOx gas-detecting elements (NOx sensor) without electrode-coatinglayers were produced. As shown in Table 1, NOx-sensing electrodes wereproduced by metal oxides active with NOx and oxygen, precious metalsactive with NOx and oxygen, and precious metals inactive with NOx andactive with oxygen. To produce a zirconia solid electrolyte substrate,green sheets of zirconia powder containing 6 mol % of yttria wereproduced by a doctor blade method. Each green sheet had a size of 0.25mm×5 mm×50 mm. Incidentally, when a sintered substrate is used, thesubstrate has a thickness of about 200 μm.

[0182] Each green sheet was cut to a rectangular shape, and a Pt leadconductor was screen-printed. Thereafter, each sensing electrodematerial shown in Table 1 was screen-printed thereon to form aNOx-sensing electrode. A Pt paste for a reference electrode wasscreen-printed onto a surface of the green sheet opposing theNOx-sensing electrode. Each of the resultant green sheets (sensorelements) for a gas-detecting element was degreased at about 500° C. inair and sintered at about 1400° C. in air. When a sintered substrate isused, a degreasing step is unnecessary, and sintering is carried out at100-1300° C. Lead wires were connected to each sintered NOx sensor toprovide a sensor sample.

[0183] Each sensor sample was set in a quartz pipe and held in anelectric furnace, in which it was exposed to a detection gas, whichcontained 100 ppm of a NOx gas (NO₂ or NO) and 5% by volume of oxygen,the balance being nitrogen, for comparison of activity with NOx. Theelectric furnace was controlled at an atmosphere temperature of 600° C.The output of each sensor sample to NOx was measured by a voltmeter withhigh input impedance. The results are shown in Table 1.

[0184] In this Reference Example, it was confirmed in advance that a Ptelectrode is not sensitive to any of NO and NO₂. TABLE 1 Sensitivity to100 Sensitivity to 100 Sensing electrode ppm of NO₂ (mV) ppm of NO (mV)A Pd 5 −2 Pt—Pd (10% by mass) 2 −1 Pd—Ru (5% by mass) 0 0 B Ir 42 −12 Au51 −13 Rh 60 −5 Ir—Au (10% by mass) 60 −15 Ir—Rh (5% by mass) 73 −16Au—Rh (5% by mass) 68 −14 B’ Pt—Rh (3% by mass) 95 −23 C NiO 78 −18 WO₃56 −11 Cr₂O₃ 96 −25 NiCr₂O₄ 103 −31 FeCr₂O₄ 97 −27 MgCr₂O₄ 95 −25 CrMnO₃70 −13 CrWO₄ 68 −12 LaCrO₃ 58 −12 NiTiO₃ 47 −11 FeTiO₃ 51 −13 ZnFe₂O₄ 61−14

[0185] As shown in Table 1, the precious metal material in the group Adid not show any sensitivity to NOx, proving that it was substantiallyinactive with NOx. On the other hand, the precious metal material in thegroup B showed high activity with NOx. Though the metal oxide materialin the group C showed excellent sensitivity to NOx, it was found thatoxides containing Cr as a constituent element among others had highsensitivity and sensitivity stability. It was found that particularlyNiCr₂O₄, FeCr₂O₄, MgCr₂O₄ and Cr₂O₃ had high sensitivity and sensitivitystability. Accordingly, it is possible to use the precious metalmaterials in the group B as the first precious metal and the preciousmetal materials in the group A as the second precious metal in thepresent invention. It was also confirmed that Pt—Rh (3% by mass) in thegroup B′, an alloy of Pt in the group A and Rh in the group B, had highsensitivity to NOx and high stability in sensitivity.

EXAMPLES 1-8

[0186] Samples of NOx gas-detecting elements (NOx sensors) having thestructure shown in FIG. 1 were produced, in the same manner as inReference Example 1 except for using an Ir—Rh (5% by mass) alloy, aPt—Rh (3% by mass) alloy, Cr₂O₃ or NiCr₂O₄ to form a NOx-sensingelectrode 5 on each solid electrolyte green sheet, and thenscreen-printing a material shown in Table 2 thereon to form anelectrode-coating layer 11 having an average thickness of 15 μm and aporosity of 30%. The reference electrode 7 was not covered by theelectrode-coating layer.

[0187] Each of the resultant sensor samples was set in a quartz pipe andheld in an electric furnace, which was controlled at an atmospheretemperature of 600° C. By an accelerated deterioration test in whicheach sample was exposed to a detection gas containing 100 ppm of NO₂ gasand 5% by volume of oxygen, the balance being nitrogen, the detectionperformance of each sensor sample was examined. An output to NO₂ wasmeasured by a voltmeter with high input impedance. The results are shownin Table 2.

Comparative Examples 1-4

[0188] Sensor samples were produced to examine their detectionperformance in the same manner as in Examples 1-8 except that noelectrode-coating layer 11 was formed. The results are shown in Table 2.TABLE 2 Initial Change Sensing Electrode-Coating Sensitivity⁽¹⁾ GasRatio of No. electrode* Layer (mV) Response⁽²⁾ Drift⁽³⁾ (%) Example 1Ir-Rh (5%) Y₂O₃ (8 mol %)- 71 ◯ −21 ZrO₂ Example 2 Ir-Rh (5%) CeO₂ (12mol %)- 70 ◯ −12 ZrO₂ Com. Ex. 1 Ir-Rh (5%) No 73 Δ −45 Example 3 Pt-Rh(3%) Y₂O₃ (8 mol %)- 90 ◯ −26 ZrO₂ Example 4 Pt-Rh (3%) CeO₂ (12 mol %)-92 ◯ −15 ZrO₂ Com. Ex. 2 Pt-Rh (3%) No 95 Δ −54 Example 5 Cr₂O₃ Y₂O₃ (8mol %)- 93 ⊚ −23 ZrO₂ Example 6 Cr₂O₃ CeO₂ (12 mol %)- 91 ⊚ −14 ZrO₂Com. Ex. 3 Cr₂O₃ No 96 ◯ −68 Example 7 NiCr₂O₄ Y₂O₃ (8 mol %)- 100 ⊚ −19ZrO₂ Example 8 NiCr₂O₄ CeO₂ (12 mol %)- 98 ⊚ −16 ZrO₂ Com. Ex. 4 NiCr₂O₄No 103 ◯ −72

[0189] The comparison of Examples 1-8 with Comparative Examples 1-4revealed that any of sensor samples of Examples 1-8 each having anelectrode-coating layer 11 had a drastically decreased change ratio ofdrift and improved response. It was confirmed that particularly when ametal oxide was used for the NOx-sensing electrode 5, the effect ofeliminating the drift was remarkable, resulting in improvement indurability.

EXAMPLES 9-16

[0190] As shown in FIG. 2, with a NOx-sensing electrode 5 fixed in arecess 1 a of a solid electrolyte substrate 1, a NOx sensor having astructure in which an electrode-coating layer 11′ made of a solidelectrolyte having gas-diffusing pores 14 was formed on an upper surfaceof the NOx-sensing electrode 5 by the following procedures: First, theNOx-sensing electrode 5 was formed in the recess 1 a formed in advancein the solid electrolyte substrate 1 by a screen-printing method. Aplurality of gas-diffusing pores 14 were formed in a zirconia greensheet having a predetermined thickness to form a green sheet for theelectrode-coating layer 11′. A green sheet for the electrode-coatinglayer 11′ was placed on the solid electrolyte substrate 1 such that thegas-diffusing pores 14 were positioned above the NOx-sensing electrode5, and the resultant assembly was pressed with lead wires inserted. Theother procedures than these steps were the same as in ReferenceExample 1. The reference electrode 7 was not covered by theelectrode-coating layer. A ratio (Sh/Se) of the total opening area (Sh)of the gas-diffusing pores 14 to the area (Se) of the sensing electrodewas set at 0.15.

[0191] Each of the resultant laminates was degreased and sintered underthe same conditions as in Reference Example 1. Any of the resultantsintered bodies had a thickness of about 30 μm. The detectionperformance of each sensor sample thus obtained was evaluated in thesame manner as in Example 1. The results are shown in Table 3.

Comparative Examples 5-8

[0192] Each sensor samples was produced to examine its detectionperformance in the same manner as in Examples 9-16 except that noelectrode-coating layer was formed. The results are shown in Table 3.TABLE 3 Sensing Initial Sensitivity⁽¹⁾ Gas Change Ratio No. electrodeElectrode-Coating Layer (mV) Response⁽²⁾ of Drift⁽³⁾ (%) Example 9 Ir—Rh(5%) Y₂O₃ (8 mol %)—ZrO₂ 69 ◯ −19 Example 10 Ir—Rh (5%) CeO₂ (12 mol%)—ZrO₂ 68 ◯ −11 Com. Ex. 5 Ir—Rh (5%) No 73 Δ −45 Example 11 Pt—Rh (3%)Y₂O₃ (8 mol %)—ZrO₂ 88 ◯ −24 Example 12 Pt—Rh (3%) CeO₂ (12 mol %)—ZrO₂91 ◯ −13 Com. Ex. 6 Pt—Rh (3%) No 95 Δ −54 Example 13 Cr₂O₃ Y₂O₃ (8 mol%)—ZrO₂ 91 ⊚ −24 Example 14 Cr₂O₃ CeO₂ (12 mol %)—ZrO₂ 90 ⊚ −11 Com. Ex.7 Cr₂O₃ No 96 ◯ −68 Example 15 NiCr₂O₄ Y₂O₃ (8 mol %)—ZrO₂ 98 ⊚ −18Example 16 NiCr₂O₄ CeO₂ (12 mol %)—ZrO₂ 96 ⊚ −14 Com. Ex. 8 NiCr₂O₄ No103 ◯ −72

[0193] The comparison of Examples 9-16 and Comparative Examples 5-8revealed that any sensor sample of Examples 9-16 provided with anelectrode-coating layer 11′ having gas-diffusing pores 14 had adrastically decreased change ratio of the drift of sensitivity ascompared with sensor samples of Comparative Examples 5-8 withoutelectrode-coating layers. In addition, the change ratio of drift wassmaller in any of the sensor samples of Examples 9-16 than in the sensorsamples of Examples 1-8 provided with porous electrode-coating layers11. On the other hand, with respect to sensitivity to 100 ppm of NO₂,the sensor samples of Examples 9-16 were slightly lower than those ofExamples 1-8, though there was no substantial decrease. With respect toresponse, it was confirmed that the sensor samples of Examples 9-16 wereimproved than those of Comparative Examples 5-8 as a whole.

EXAMPLES 17-28

[0194] Samples of NOx sensors having the structures shown in FIGS. 3, 4,6, 8-10 were produced. Cr₂O₃ or NiCr₂O₄ was used for a sensingelectrode, and a zirconia solid electrolyte containing 12 mol % of CeO₂was used for an electrode-coating layer.

[0195] Sensor samples having the structure shown in FIG. 3 wereproduced, in the same manner as in Reference Example 1 except that azirconia solid electrolyte containing 14 mol % of CeO₂ wasscreen-printed on a green sheet for a solid electrolyte substrate, toform an electrode underlayer 31 having a porosity of 10% and a thicknessof 3 μm, and that after forming a NOx-sensing electrode 5, anelectrode-coating layer 11 having a porosity of 30% and an averagethickness of 15 μm was formed by a screen-printing method.

[0196] Sensor samples having the structure shown in FIG. 4 wereproduced, in the same manner as in Reference Example 1 except forforming an alumina print layer as an electric insulating layer 32 on agreen sheet for the solid electrolyte substrate 1, forming a NOx-sensingelectrode 5 and a reference electrode 7, and then screen-printing anelectrode-coating layer 11 thereon.

[0197] Sensor samples having the structure shown in FIG. 6 wereproduced, in the same manner as in Reference Example 1 except that aNOx-sensing electrode 5 and a reference electrode 7 were formed on thesame surface of the green sheet, and that an electrode-coating layer 11was screen-printed only on the NOx-sensing electrode 5.

[0198] Sensor samples having the structure shown in FIG. 8 wereproduced, in the same manner as in Reference Example 1 except that a300-μm-thick, high-purity alumina substrate was used as an electricinsulating substrate 41, that a zirconia print layer (solid electrolytesubstrate layer) 1′ containing 6 mol % of yttria was formed thereon,that a NOx-sensing electrode 5 and a reference electrode 7 were thenformed on the same surface of the green sheet, and that anelectrode-coating layer 11 was screen-printed only on the NOx-sensingelectrode 5.

[0199] Sensor samples having the structure shown in FIG. 9 wereproduced, in the same manner as in Reference Example 1 except that aftera first NOx-sensing electrode 5 a was formed on the zirconia solidelectrolyte substrate 1, an electrode-coating layer 11 and a secondNOx-sensing electrode 5 b were successively screen-printed. The firstNOx-sensing electrode 5 a and the second NOx-sensing electrode 5 b wereformed by the same electrode material.

[0200] Sensor samples having the structure shown in FIG. 10 wereproduced, in the same manner as in Reference Example 1 except that aftera first NOx-sensing electrode 5 a was formed on the zirconia solidelectrolyte substrate 1, an electrode-coating layer 11 and a secondNOx-sensing electrode 5 b were printed, and that a furtherelectrode-coating layer 11 was screen-printed thereon. The firstNOx-sensing electrode 5 a and the second NOx-sensing electrode 5 b wereformed by the same electrode material.

[0201] With respect to each of the resultant sensor samples, sensitivitywas evaluated in the same manner as in Example 1. The results are shownin Table 4. Incidentally, for the comparison of performance, thesensitivity of the sensor samples having the structure shown in FIG. 1(Examples 6, 8) was also shown in Table 4. TABLE 4 Change Initial Ratioof Sensing Sensitivity⁽¹⁾ Gas Drift⁽³⁾ No. Structure electrode (mV)Response⁽²⁾ (%) Ex. 8 NiCr₂O₄ 98 ⊚ −16 Ex. 6 Cr₂O₃ 91 ⊚ −14 Ex. 17NiCr₂O₄ 102 ⊚ −12 Ex. 18 Cr₂O₃ 96 ⊚ −11 Ex. 19 NiCr₂O₄ 91 ⊚⊚ −17 Ex. 20Cr₂O₃ 86 ⊚⊚ −18 Ex. 21 NiCr₂O₄ 97 ⊚ −16 Ex. 22 Cr₂O₃ 92 ⊚ −17 Ex. 23NiCr₂O₄ 96 ⊚ −18 Ex. 24 Cr₂O₃ 94 ⊚ −16 Ex. 25 NiCr₂O₄ 95 ⊚⊚ −20 Ex. 26Cr₂O₃ 87 ⊚⊚ −18 Ex. 27 NiCr₂O₄ 99 ⊚⊚ −18 Ex. 28 Cr₂O₃ 92 ⊚⊚ −16

[0202] As compared with the sensor samples having the structure shown inFIG. 1, the sensor samples having the structure shown in FIG. 3(Examples 17 and 18) had decreased change ratios of drift, confirmingthe effect of providing the electrode underlayer 31. Though the sensorsamples having the structure shown in FIG. 4 (Examples 19 and 20)slightly decreased in sensitivity, they were excellent in gas response.It is presumed that this effect is obtained by the fact that because ofthe formation of the electric insulating layer 32, there is an electrodeinterface, in which a gas detection reaction occurs, on the uppersurface of the sensing electrode 5.

[0203] The sensor samples having the structure shown in FIG. 6 (Examples21 and 22) showed substantially the same performance as that of thesensor samples having the structure shown in FIG. 1. This confirmed thatthe same effect was obtained when the sensing electrode 5 and thereference electrode 7 were formed on one surface of the solidelectrolyte substrate 1 and when they were formed on both surfacesthereof.

[0204] The sensor samples having the structure shown in FIG. 8 (Examples23 and 24) showed substantially the same performance as that of thesensor samples having the structure shown in FIG. 6. This confirmed thatthe same performance was obtained when the thin solid electrolytesubstrate layer 1′ was formed on the electric insulating substrate 41and when the solid electrolyte substrate 1 was used.

[0205] The sensor samples having the structure shown in FIGS. 9 and 10(Examples 25-28) were superior to the sensor samples having thestructure shown in FIG. 1 in gas response. This is presumed to be due tothe fact that the formation of two sensing electrodes increased theelectrode interface area. In addition, the sensor samples having thestructure shown in FIGS. 9 and 10 were improved in response whilesubstantially maintaining excellent sensitivity and stability, like thesensor samples having the structure shown in FIG. 4.

[0206] It is clear from the above results that the sensor samples havingthe structures of the present invention have small drift of sensitivityand are not only excellent in the stability of sensitivity but alsoimproved in response.

EXAMPLES 29-35

[0207] Samples of NOx sensors having the structure shown in FIG. 1 wereproduced in the same manner as in Reference Example 1 except for usingNiCr₂O₄ for NOx-sensing electrodes 5, and using zirconia solidelectrolyte layers containing 10 mol % of various stabilizers shown inTable 5 for electrode-coating layers 11. The electrode-coating layers 11of the resultant NOx sensors had a porosity of 30% and an averagethickness of 15 μm. Each of the resultant sensor samples was evaluatedwith respect to performance in the same manner as in Example 1. Theresults are shown in Table 5. For comparison, the data of ComparativeExample 4 are also shown. TABLE 5 Initial Gas Change Ratio No.Electrode-Coating Layer Sensitivity⁽¹⁾ (mV) Response⁽²⁾ of Drift⁽³⁾ (%)Example 29 Y₂O₃ (10 mol %)—ZrO₂ 101 ◯ −21 Example 30 CaO (10 mol %)—ZrO₂91 ⊚ −38 Example 31 MgO (10 mol %)—ZrO₂ 95 ⊚ −19 Example 32 CeO₂ (10 mol%)—ZrO₂ 103 ⊚ −16 Example 33 Sc₂O₃ (10 mol %)—ZrO₂ 98 ⊚ −17 Example 34ThO₂ (10 mol %)—ZrO₂ 86 ◯ −34 Example 35 Yb₂O₃ (10 mol %)—ZrO₂ 90 ◯ −29Com. Ex. 4 No 103 ◯ −72

[0208] As shown in Table 5, remarkable reduction effect of the changeratio of drift was observed in Examples 29-35 than in ComparativeExample 9, irrespective of the materials of the electrode-coating layers11. Such effect was remarkable particularly when a zirconia solidelectrolyte layer containing Y₂O₃, MgO, CeO₂ or Sc₂O₃ as a stabilizerwas used.

EXAMPLES 36-42

[0209] Samples of the NOx sensors of the present invention having thestructure shown in FIG. 3 were produced, in the same manner as inReference Example 1 except for using NiCr₂O₄ for a NOx-sensing electrode5 and a zirconia solid electrolyte containing 12 mol % of CeO₂ forelectrode-coating layers 11, using zirconia solid electrolytescontaining 10 mol % of various stabilizers shown in Table 6 for anelectrode underlayer 31 having a porosity of 10% and a thickness of 3μm, and screen-printing an electrode underlayer 31, a NOx-sensingelectrode 5 and an electrode-coating layer 11 (porosity: 30%, averagethickness: 15 μm), respectively, on the solid electrolyte substrate 1.The detection performance of each of the resultant sensor samples wasevaluated in the same manner as in Example 1. The results are shown inTable 6. TABLE 6 Initial Gas Change Ratio No. Electrode UnderlayerSensitivity⁽¹⁾ (mV) Response⁽²⁾ of Drift⁽³⁾ (%) Example 36 Y₂O₃(10 mol%)—ZrO₂ 102 ⊚ −10 Example 37 CaO(10 mol %)—ZrO₂ 93 ⊚ −14 Example 38MgO(10 mol %)—ZrO₂ 94 ⊚ −10 Example 39 CeO₂(10 mol %)—ZrO₂ 105 ⊚ −6Example 40 Sc₂O₃(10 mol %)—ZrO₂ 99 ⊚ −9 Example 41 ThO₂(10 mol %)—ZrO₂84 ⊚ −13 Example 42 Yb₂O₃(10 mol %)—ZrO₂ 91 ⊚ −12

[0210] As shown in Table 6, the sensor samples of Examples 36-42 wereremarkably improved in gas response and the suppression of driftirrespective of the materials of the electrode underlayer 31.Particularly when the same CeO₂-added zirconia solid electrolyte as inthe electrode-coating layer 11 was used for the electrode underlayer 31,the electrode underlayer 31 exhibited large effects.

EXAMPLES 43-54

[0211] Samples of NOx sensors having the structure shown in FIG. 1 wereproduced, in the same manner as in Reference Example 1 except for usingmetal oxides shown in Table 7 for the NOx-sensing electrode 5, and azirconia solid electrolyte layer containing 12 mol % of CeO₂ (porosity:30%, average thickness: 15 μm) for the electrode-coating layer 11. Eachof the resultant sensor samples was evaluated with respect toperformance in the same manner as in Example 1. The results are shown inTable 7. TABLE 7 Change Ratio of Drift Initial No Coating With CoatingSensing Sensitivity⁽¹⁾ Gas Layer Layer No. electrode (mV) Response⁽²⁾ E₀⁽³⁾ (%) E₁ ⁽³⁾ (%) E₁/E₀ Example 43 NiO 72 Δ −79 −38 0.48 Example 44 WO₃64 ◯ −82 −40 0.49 Example 45 Cr₂O₃ 95 ⊚ −68 −15 0.22 Example 46 NiCr₂O₄102 ⊚ −72 −15 0.21 Example 47 FeCr₂O₄ 94 ⊚ −75 −19 0.25 Example 48MgCr₂O₄ 93 ◯ −73 −17 0.23 Example 49 CrMnO₃ 75 ◯ −75 −22 0.29 Example 50CrWO₄ 69 ◯ −78 −25 0.32 Example 51 LaCrO₃ 62 ◯ −81 −27 0.33 Example 52NiTiO₃ 43 Δ −85 −36 0.42 Example 53 FeTiO₃ 50 Δ −83 −34 0.41 Example 54ZnFe₂O₄ 65 Δ −85 −33 0.39

[0212] As shown in Table 7, when metal oxides containing Cr as aconstituent element were used among the sensing electrode materials, NOxsensors with small change ratios of drift were obtained. Particularlywhen the NOx-sensing electrode 5 was made of Cr₂O₃, NiCr₂O₄, FeCr₂O₄ orMgCr₂O₄, there was a large effect of reducing the change ratio of drift.

EXAMPLES 55-66

[0213] Samples of NOx sensors having the structure shown in FIG. 1 wereproduced, in the same manner as in Reference Example 1 except for usingNiCr₂O₄ for the NOx-sensing electrode 5 and a zirconia solid electrolytelayer containing 12 mol % of CeO₂ (porosity: 30%, average thickness: 15μm) for the electrode-coating layer 11. As shown in Table 8, the firstprecious metal (in the group B) active with NOx and oxygen, the secondprecious metal (in the group A) active with only oxygen, or Pt—Rh (3% bymass), an alloy of the first and second precious metals (in the groupB′), was added to the electrode-coating layer 11 in an amount of 1.0% bymass. The detection performance of each of the resultant sensor sampleswas evaluated in the same manner as in Example 1. The results are shownin Table 8. TABLE 8 Initial Change Precious Metal in Sensitivity⁽¹⁾ GasRatio No. Electrode-Coating Layer (mV) Response⁽²⁾ of Drift⁽³⁾ (%)Example 55 No 102 ◯ −15 Example 56 A Pt 90 ⊚ −8 Example 57 Pd 86 ⊚ −11Example 58 Pt-Pd (10% by mass) 91 ⊚ −12 Example 59 Pd-Ru (5% by mass) 97⊚ −14 Example 60 B Ir 103 ◯ −10 Example 61 Au 105 ◯ −11 Example 62 Rh109 ◯ −9 Example 63 Ir-Au (10% by mass) 103 ◯ −13 Example 64 Ir-Rh (5%by mass) 106 ◯ −11 Example 65 Au-Rh (5% by mass) 108 ◯ −15 Example 66 B’Pt-Rh (3% by mass) 112 ◯ −16

[0214] As shown in Table 8, as compared with the sample (Example 55) inwhich the electrode-coating layer 11 did not contain a precious metal,the samples containing the second precious metals in the group A hadgreatly improving gas response while suppressing the drift, though theyexhibited slightly decreased sensitivity. The samples containing thefirst precious metals in the group B had the same or improved gassensitivity while suppressing the drift, though there was no improvementin gas response. The sample containing Pt—Rh, an alloy of the first andsecond precious metals, was improved in sensitivity and its responsewhile suppressing the drift.

EXAMPLES 67 AND 68

[0215] Samples of NOx sensors having the structures shown in FIGS. 1 and2 were produced, in the same manner as in Example 1 except for usingNiCr₂O₄ for the NOx-sensing electrode 5, and a zirconia solidelectrolyte layer (average thickness: 20 μm) containing 12 mol % of CeO₂for the electrode-coating layer 11, 11′. The electrode-coating layer 11was a porous layer having a porosity of 40%, while the electrode-coatinglayer 11′ was a dense layer having a porosity of 0.5%. The sensor samplehaving the structure shown in FIG. 1 was produced, in the same manner asin Reference Example 1, except that after a NOx-sensing electrode 5 wasformed on a green sheet for the solid electrolyte substrate 1, anelectrode-coating layer 11 was formed by a screen-printing method. Thesensor sample having the structure shown in FIG. 2 was produced in thesame manner as in Reference Example 1, except that after a NOx-sensingelectrode 5 was formed on a green sheet for the solid electrolytesubstrate 1, a zirconia solid electrolyte sheet containing 6 mol % ofY₂O₃ was punched to form an electrode-coating layer 11′ havinggas-diffusing pores 14 having Sh/Se of about 0.2, laminating theelectrode-coating layer 11′ on the solid electrolyte substrate 1 suchthat it covered the sensing electrode 5, and pressing the resultantlaminate to bond the substrate layers. The thickness of theelectrode-coating layers 11, 11′ was changed to various levels as shownin Table 9. The detection performance of each of the resultant sensorsamples was evaluated in the same manner as in Example 1. The resultsare shown in Table 9. TABLE 9 Change Thickness⁽⁴⁾ of Initial Ratio ofElectrode-Coating Sensitivity⁽¹⁾ Gas Drift⁽³⁾ No. Layer⁽⁵⁾ (μm) (mV)Response⁽²⁾ (%) Com. Ex. 4 0 103 ◯ −72 Example 67 1.9 93 ◯ −48 2.8 89 ⊚−24 5.6 88 ⊚ −21 10.8 86 ⊚ −19 20.5 88 ⊚ −17 28.4 82 ◯ −16 34.1 75 Δ −17Example 68 3.5 94 ◯ −46 5 93 ⊚ −21 26 95 ⊚ −17 51 92 ⊚ −16 72 90 ⊚ −15105 88 ⊚ −16 155 73 Δ −15

[0216] As is clear from the comparison of Example 67 with ComparativeExample 10 and the comparison of Example 68 with Comparative Example 11,any of the porous electrode-coating layers 11 and the electrode-coatinglayers 11′ with gas-diffusing pores was improved in any of initialsensitivity, gas response and the change ratio of drift, when they had aproper thickness. It is clear that in the case of the porouselectrode-coating layer 11, its thickness is preferably 2.8-20.5 μm, andthat in the case of the electrode-coating layer 11′ with gas-diffusingpores, its thickness is preferably 5-105 μm.

EXAMPLES 69 AND 70

[0217] Samples of NOx sensors having the structure shown in FIG. 1 wereproduced, in the same manner as in Example 67 except that the thicknessof the electrode-coating layer 11 was as constant as about 5 μm, andthat its porosity was changed as shown in Table 10. Also, samples of NOxsensors having the structure shown in FIG. 2 were produced, in the samemanner as in Example 68 except that the dense electrode-coating layer11′ (porosity: 0.5%) having 50 gas-diffusing pores 14 had a constantthickness of about 50 μm, and that a ratio of (Sh/Se) of the totalopening area (Sh) of the gas-diffusing pores 14 to the area (Se) of thesensing electrode 5 was changed as shown in Table 10. By evaluating thedetection performance of these sensor samples in the same manner as inExample 1, the influence of the porosity of the porous electrode-coatinglayer 11 on sensor characteristics (Example 69), and the influence ofSh/Se of the electrode-coating layer 11′ with gas-diffusing pores onsensor characteristics (Example 70) were examined. The evaluationresults of Example 69 are shown in Table 10, and the evaluation resultsof Example 70 are shown in Table 11.

Comparative Examples 9 and 10

[0218] Samples of NOx sensors (gas-detecting elements) were produced toevaluate detection performance in the same manner as in Example 69except that the porosity of the porous electrode-coating layer 11 was 4%and 59%, respectively. The results are shown in Table 10.

Comparative Examples 11 and 12

[0219] Samples of NOx sensors were produced to evaluate detectionperformance in the same manner as in Example 70 except that the Sh/Se ofthe electrode-coating layer 11′ having gas-diffusing pores 14 was 3% and33%, respectively. The results are shown in 11. TABLE 10 InitialPorosity Sensitivity⁽¹⁾ Gas Change Ratio No. (%) (mV) Response⁽²⁾ ofDrift⁽³⁾ (%) Com. Ex. 9 4 X⁽⁴⁾ X⁽⁴⁾ X⁽⁴⁾ Example 69 10 86 ◯ −17 22 89 ⊚−19 31 91 ⊚ −16 38 88 ⊚ −19 51 85 ⊚ −23 Com. Ex. 10 59 82 ◯ −49

[0220] TABLE 11 Initial Sensitivity⁽¹⁾ Gas Change Ratio No. Sh/Se (%)(mV) Response⁽²⁾ of Drift⁽³⁾ (%) Com. Ex. 11 3 74 Δ −19 Example 70 5 92◯ −16 12 90 ⊚ −18 16 93 ⊚ −16 23 92 ⊚ −19 28 92 ⊚ −22 Com. Ex. 12 33 84⊚ −36

[0221] As shown in Tables 10 and 11, there are optimum ranges, forsensor characteristics, in both of the porosity of the porouselectrode-coating layer 11 and the Sh/Se of the electrode-coating layer11′ with gas-diffusing pores, respectively. In the case of the porouselectrode-coating layer 11, good sensitivity and stability are obtainedwhen the porosity is in a range of 10-51%, and response is improved whenthe porosity is restricted to 22-51%. On the other hand, in the case ofthe electrode-coating layer 11′ with gas-diffusing pores, excellentsensitivity and stability are obtained when the Sh/Se is in a range of5-28%, and response is improved when the Sh/Se is restricted to 12-28%.

EXAMPLES 71-83 Comparative Examples 13, 14

[0222] Samples of NOx gas-detecting elements (NOx sensors) shown inFIGS. 11 and 12 were produced. Zirconia solid electrolyte green sheetsof 5 mm×5 mm×0.25 mm were produced using zirconia powder containing 6mol % of yttria by a doctor blade method. Each of the resultant greensheets was screen-printed with a Pt lead conductor, a NOx-sensingelectrode 5, a reference electrode 7 and an electrode-coating layer 12.The NOx-sensing electrode 5 was made of NiCr₂O₄, and its size was 2 mm×2mm×0.003 mm. The reference electrode 7 was made of Pt or an alloy of Ptcontaining 1% by mass of Rh, and screen-printed on a surface of thezirconia solid electrolyte substrate 1 opposing the NOx-sensingelectrode 5. The material, shape and porosity of the reference electrode7 and the electrode-coating layer 12 covering the reference electrode 7are as shown in Table 12.

[0223] It is known that though Pt per se has substantially no activitywith NOx, alloys of Pt+1% by mass Rh are active with NOx. Accordingly,to examine whether or not the reference electrode 7 becomes inactivewith NOx when the electrode-coating layer 12 is formed, a Pt—Rh alloywas used for the reference electrode 7.

[0224] Samples of gas-detecting elements each having a referenceelectrode 7 formed by Pt or Pt+1% by mass Rh with no electrode-coatinglayer were produced in Comparative Examples 16, 17. The referenceelectrode 7 had a size of 2 mm×2 mm×0.003 mm.

[0225] A green sheet laminate for each gas-detecting element wasdegreased at 500° C. for 2 hours in air, and sintered at 1400° C. for 3hours in air. Lead wires were connected to each of the resultantsintered bodies to provide samples of NOx sensors.

[0226] Each sensor sample thus produced was set in a quartz pipe andheld in an electric furnace, in which the NOx-sensing electrode 5 andthe reference electrode 7 were exposed to a detection gas containing 100ppm of NO₂ and 5% by volume of oxygen, the balance being nitrogen, toexamine its activity with NOx. The electric furnace was controlled at anatmosphere temperature of 600° C. The output of each sample was measuredby a voltmeter with high input impedance, and the sensitivity of eachsample was evaluated by the difference in output between a base gascontaining 5% by volume of oxygen, the balance being nitrogen, and adetection gas (obtained by adding 100 ppm of NO₂ to the base gas).Interface impedance between the reference electrode 7 and the solidelectrolyte substrate 1 was measured by an impedance analyzer. Theresults are shown in Table 13. TABLE 12 Electrode-Coating LayerReference Size Porosity No. Electrode Material (mm) (vol. %) Com. Ex. 13Pt No — — Com. Ex. 14 Pt—1% by mass Rh No — — Ex. 71⁽¹⁾ Pt CeO₂ (12 mol%)—ZrO₂   3 × 3 × 0.01 30 Ex. 72⁽¹⁾ Pt—1% by mass Rh CeO₂ (12 mol%)—ZrO₂   3 × 3 × 0.01 30 Ex. 73⁽¹⁾ Pt—1% by mass Rh Y₂O₃ (8 mol %)—ZrO₂  3 × 3 × 0.01 30 Ex. 74⁽¹⁾ Pt—1% by mass Rh MgO (15 mol %)—ZrO₂   3 × 3× 0.01 30 Ex. 75⁽¹⁾ Pt—1% by mass Rh Sc₂O₃ (12 mol %)—ZrO₂   3 × 3 ×0.01 30 Ex. 76⁽¹⁾ Pt—1% by mass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.0110 Ex. 77⁽¹⁾ Pt—1% by mass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.01 20 Ex.78⁽¹⁾ Pt—1% by mass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.01 50 Ex. 79⁽¹⁾Pt—1% by mass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.003 30 Ex. 80⁽¹⁾ Pt—1%by mass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.015 30 Ex. 81⁽¹⁾ Pt—1% bymass Rh CeO₂ (12 mol %)—ZrO₂   3 × 3 × 0.02 30 Ex. 82⁽²⁾ Pt—1% by massRh CeO₂ (12 mol %)—ZrO₂ 2.5 × 3 × 0.005  0 Ex. 83⁽²⁾ Pt—1% by mass RhY₂O₃ (3 mol %)—ZrO₂ 2.5 × 3 × 0.005  0

[0227] TABLE 13 Sensitivity to 100 Interface No. ppm of NO₂ (mV)Impedance (kΩ) Com. Ex. 13 103 20 Com. Ex. 14 75 50 Example 71⁽¹⁾ 110 10Example 72⁽¹⁾ 97 25 Example 73⁽¹⁾ 98 24 Example 74⁽¹⁾ 95 26 Example75⁽¹⁾ 94 27 Example 76⁽¹⁾ 100 23 Example 77⁽¹⁾ 98 24 Example 78⁽¹⁾ 95 27Example 79⁽¹⁾ 95 25 Example 80⁽¹⁾ 98 26 Example 81⁽¹⁾ 99 24 Example82⁽²⁾ 101 21 Example 83⁽²⁾ 102 20

[0228] The sample of Comparative Example 16 having a reference electrode7 made of Pt had sensitivity of 103 mV to 100 ppm of NO₂, and interfaceimpedance of 20 kΩ. The sample of Comparative Example 17 having areference electrode 7 made of an alloy of Pt and 1% by mass of Rh activewith NOx had as large interface impedance as 50 kΩ, and as smallsensitivity as 75 mV. This is presumed to be due to the fact thatbecause the potential of an electrode made of an alloy of Pt and 1% bymass of Rh is about 30 mV in the same direction as the potential of theNOx-sensing electrode 5, the sensitivity corresponding to potentialdifference between both electrodes becomes smaller accordingly.

[0229] Examined in Example 71 were the characteristics of thegas-detecting element sample having an electrode-coating layer 12 madeof porous ceria-stabilized zirconia on a Pt reference electrode 7. Thesample of Example 71 had interface impedance of 10 kΩ, which was abouthalf of that of Comparative Example 1 free from an electrode-coatinglayer 12, and as large sensitivity as 110 mV. It is presumed that thoughpure Pt had substantially no sensitivity to NOx, the Pt electrode of thegas-detecting element of Comparative Example 1 that did not have anelectrode-coating layer 12 became active with NOx by contamination, etc.during its production processes, resulting in decrease in sensitivity.On the other hand, why the sensitivity is higher in Example 71 than inComparative Example 1 is presumed to be due to the fact that the coatinglayer 12 formed on the Pt electrode suppressed contamination during theproduction processes, thereby keeping the activity with NOxsubstantially zero.

[0230] Examined in Examples 72-75 was the influence of a stabilizeradded to an electrode-coating layer 12 made of a zirconia solidelectrolyte. In any Examples, the reference electrode 7 was made of analloy of Pt and 1% by mass of Rh, and the porous electrode-coating layer12 having a size of 3 mm×3 mm×0.01 mm and a porosity of 30% by volumecompletely covered the reference electrode 7. Though there are somedifferences in interface impedance and sensitivity depending on thetypes of stabilizers, any of Examples 72-75 exhibited smaller interfaceimpedance with larger sensitivity by about 20 mV than ComparativeExample 2. This is presumed to be due to the fact that because thedetection gas in this Example had a sufficiently high oxygenconcentration as compared with the concentration of NOx, a detectionobject gas, decrease in the interface impedance results in increase onlyin the reaction sites of oxygen without substantially changing thereaction sites of NOx, the activity of the reference electrode to NOxdecreased.

[0231] In Examples 72 and 76-78, the influence of the porosity ofelectrode-coating layer 12 was examined. In any Examples, theelectrode-coating layer 12 having a size of 3 mm×3 mm×0.01 mm completelycovered the reference electrode 7. When the electrode-coating layer 12had porosity in a range of 10-50%, they exhibited substantially the sameinterface impedance as that of Comparative Example 2 and largersensitivity than that of Comparative Example 2 by about 20 mV or more,confirming that the activity of the reference electrode 7 to NOx wasdecreased.

[0232] In Examples 72 and 79-81, the influence of the thickness of theporous electrode-coating layer 12 was examined. Within the thicknessrange of these Examples, they exhibited substantially the same interfaceimpedance as that of Comparative Example 17 and larger sensitivity thanthat of Comparative Example 17 by 20 mV or more, confirming that theactivity of the reference electrode 7 to NOx was decreased.

[0233] Examined in Examples 82 and 83 was the sensitivity of sensorelements having the structure shown in FIG. 12, when a denseelectrode-coating layer 12 was laminated on a reference electrode 7.Used for the electrode-coating layer 12 was zirconia stabilized by 12mol % of ceria in Example 82, and zirconia stabilized by 3 mol % ofyttria in Example 83. In any Examples, the electrode-coating layer 12had a size of 2.5 mm×3 mm×0.005 mm, and was not formed on one of thefour side surfaces of the reference electrode 7. The interface impedancewas 21 kΩ in Example 82 and 20 kΩ in Example 83, remarkably smaller thanthat of Comparative Example 17. The sensitivity to 100 ppm of NO₂ was101 mV in Example 82, larger than that of Comparative Example 17 by 26mV, and 102 mV in Example 83, larger than that of Comparative Example 17by 27 mV. This proved that the activity of the reference electrode 7 toNOx was decreased by the dense electrode-coating layer 12. Substantiallyno deterioration in response was observed by the dense coating layer 12.

EXAMPLES 84, 85 Comparative Examples 15, 16

[0234] Samples of NOx sensors having the structure shown in FIG. 7 wereproduced, in the same manner as in Reference Example 1 except that Cr₂O₃or NiCr₂O₄ was used for a NOx-sensing electrode 5, that Pt was used fora reference electrode 7, that a zirconia solid electrolyte containing 12mol % of CeO₂ was used for electrode-coating layers 11, 12, and thatelectrode-coating layers 11, 12 (porosity: 30%, average thickness: 15μm) were screen-printed on the NOx-sensing electrode 5 and the referenceelectrode 7. The performance of the resultant samples was measured inthe same manner as in Examples 17-28. Also, with respect to the sensorsamples of Comparative Examples 15, 16, which were the same as those ofExamples 84, 85 except that no electrode-coating layer was formed on anyof the NOx-sensing electrode and the reference electrode, performancewas measured in the same manner as in Examples 17-28. The results areshown in Table 14.

[0235] For comparison, the measurement results of the sensor samplesshown in FIG. 6 (Examples 21, 22), in which the electrode-coating layer11 was formed only on the NOx-sensing electrode were also shown in Table14. TABLE 14 Sensing Initial gas Change Ratio No. Structure electrodeSensitivity⁽¹⁾ (mV) Response⁽²⁾ of Drift⁽³⁾ (%) Example 84 NiCr₂O₄ 96 ◯−12 Example 85 Cr₂O₃ 93 ◯ −13 Com. Ex. 15 — NiCr₂O₄ 103 — −73 Com. Ex.16 — Cr₂O₃ 96 — −68 Example 21 NiCr₂ O₄ 97 ◯ −16 Example 22 Cr₂O₃ 92 ◯−17

[0236] It was found that the sensor samples having the structure shownin FIG. 7 (Examples 84 and 85) had a decreased change ratio of driftthan that of the sample having the structure shown in FIG. 6. This ispresumed to be due to the fact that the stabilization of the referenceelectrode 7 contributed to decrease in the drift of sensor output.

EXAMPLES 86-94

[0237] Samples of laminate-type NOx gas-detecting devices having thestructures shown in FIGS. 14 and 15 were produced. Cr₂O₃ or NiCr₂O₄ wasused for a NOx-sensing electrode 5, and a zirconia solid electrolytelayer containing 12 mol % of CeO₂ (porosity: 30%, average thickness 15μm) or a zirconia solid electrolyte layer containing 8 mol % of Y₂O₃(porosity: 30%, average thickness 15 μm) was used for electrode-coatinglayers 11, 12. Samples having electrode-coating layers 11, to which Rh(a first precious metal) and/or Pt (a second precious metal) was addedin an amount of 0.5% by mass based on the electrode-coating layer, werealso produced. A green sheet for the solid electrolyte substrate 1 wasproduced by a doctor blade method using zirconia powder containing 6 mol% of yttria.

[0238] Formed on the green sheet I for the solid electrolyte substrate 1by a screen-printing method to form a NOx-detecting cell were aNOx-sensing electrode 5, a Pt reference electrode 7, electrode-coatinglayers 11, 12, and Pt lead conductors. Also, a green sheet II having thesame composition was screen-printed with a NOx-converting electrode 8made of Pt—Rh and a conversion counter electrode 9 made of Pt, to form aNOx-converting pump element. Further, a heater was sandwiched by twogreen sheets III having the same composition to form a heater portion.These green sheets I-III were laminated with green sheets for formingair ducts, and green sheets for forming the air ducts, spacers and a gasinlet.

[0239] With portions for forming internal spaces (a gas-measuringchamber, air ducts, etc.) filled with theobromine, which was sublimed ata degreasing step, each laminate was pressed while heating. After eachlaminate was degreased, it was at sintered 1400° C. Each of theresultant sintered laminates was provided with lead wires to constitutea laminate-type NOx sensor.

[0240] Each laminate-type NOx sensor sample was connected to a controlunit, set in a quartz pipe and held in an electric furnace. A detectiongas containing 50 ppm of NO, 50 ppm of NO₂ and 5% by volume oxygen, thebalance being nitrogen was caused to flow through the quartz pipe. Withthe atmosphere temperature of the electric furnace controlled to 600°C., the NOx-detecting performance of each sensor sample was evaluatedwhile applying a predetermined potential to the conversion pump elementin the same manner as in Example 1. The compositions of the electrodesare shown in Table 15, and the measurement results are shown in Table16.

Comparative Examples 17, 18

[0241] Laminate-type sensor samples were produced to evaluate theirdetection performance, in the same manner as in Examples 86-94 exceptthat an electrode-coating layer was not formed. The compositions of theelectrodes are shown in Table 15, and the measurement results are shownin Table 16. TABLE 15 Electrode-Coating Layer First Second SensingPrecious Precious No. electrode Metal Oxide Metal Metal Example 86 Cr₂O₃CeO₂(12 mol %)-ZrO₂ Rh No Example 87 Cr₂O₃ CeO₂(12 mol %)-ZrO₂ No PtExample 88 Cr₂O₃ CeO₂(12 mol %)-ZrO₂ Rh Pt Com. Ex. 17 Cr₂O₃ No No NoExample 89 NiCr₂O₄ CeO₂(12 mol %)-ZrO₂ Rh No Example 90 NiCr₂O₄ CeO₂(12mol %)-ZrO₂ No Pt Example 91 NiCr₂O₄ CeO₂(12 mol %)-ZrO₂ Rh Pt Com. Ex.18 NiCr₂O₄ No No No Example 92 Cr₂O₃ Y₂O₃(8 mol %)-ZrO₂ Rh No Example 93Cr₂O₃ Y₂O₃(8 mol %)-ZrO₂ No Pt Example 94 Cr₂O₃ Y₂O₃(8 mol %)-ZrO₂ Rh Pt

[0242] TABLE 16 Element of FIG. 14 Element of FIG. 15 Initial ChangeInitial Sensitivity⁽¹⁾ Ratio of Sensitivity⁽¹⁾ Change Ratio No. (mV)Drift⁽²⁾ (%) (mV) of Drift⁽²⁾ (%) Example 86 40 −26 38 −29 Example 87 38−22 36 −20 Example 88 42 −18 40 −19 Com. Ex. 17 37 −49 35 −54 Example 8941 −28 41 −29 Example 90 43 −20 42 −23 Example 91 40 −22 38 −20 Com. Ex.18 42 −51 33 −56 Example 92 42 −28 39 −25 Example 93 40 −19 39 −24Example 94 39 −19 36 −23

[0243] As shown in Table 16, any laminate-type sensor structures can beprovided with drastically decreased change ratios of drift by formingelectrode-coating layers, like the single layer sensor (elementstructure, etc. shown in FIG. 1) in Example 1, etc.

EXAMPLES 95-110 Comparative Example 19

[0244] Gas-detecting devices (laminate-type NOx sensors) having thestructure shown in FIGS. 15-18 were produced. Green sheets of 6 mm×70 mmfor solid electrolyte substrates were produced from zirconia powdercontaining 6 mol % of yttria by a doctor blade method. The thickness ofeach green sheet was 0.1-0.3 mm, though it changed depending onportions. Each green sheet was screen-printed with Pt lead conductors, aNOx sensing electrode, a reference electrode, an oxygen-sensingelectrode and its electrode-coating layer, a NOx-converting electrode, aNOx-converting counter electrode, a gas-treating electrode, and aheater. These green sheets were laminated, degreased at 500° C. for 2hours, and sintered at 1400° C. for 3 hours in the air to providesamples.

[0245] The NOx-sensing electrode 5 was made of NiCr₂O₄, having a size of0.7 mm×1.3 mm×0.003 mm. The reference electrode 7 shown in FIG. 15 andthe oxygen-sensing electrode 6 shown in FIGS. 17 and 18, each having asize of 0.7 mm×1.8 mm×0.005 mm, were made of Pt, an alloy of Pt and 50%by mass of Ir, or an alloy of Pt and 1% by mass of Au, respectively. Thereference electrodes 7 shown in FIGS. 17 and 18 were made of Pt, havinga size of 0.7 mm×1.3 mm×0.003 mm.

[0246] The NOx-converting electrode 8 was made of an alloy of Pt and 3%by mass of Rh, the NOx-converting counter electrode 9 was made of Pt,and the gas-treating electrode 10 was made of Pt. Zirconia stabilized by10% by mass of yttria (8 mol %) was added to each electrode to produce agas electrode. The material, size, porosity and form of theelectrode-coating layer 12 covering the reference electrode 7 in FIG.15, and the material, size, porosity and form of the electrode-coatinglayer 13 covering the oxygen-sensing electrode 6 in FIGS. 17 and 18 areas shown in Table 17.

[0247] Each of the resultant sensor samples was charged into an aluminatube, which was set in a measurement jig, which was then assemble din agas sensitivity evaluation apparatus. Each sensor sample was exposed toa detection gas containing 100 ppm of an NO gas and 5% by volume oxygen,the balance being nitrogen. Voltage of 0.8 V was applied to theNOx-converting pump in such a direction that oxygen is introduced intothe gas-measuring chamber, to convert NO to NO₂. In the gas-detectingdevice shown in FIG. 18, a constant voltage of 0.8 V was applied to thegas-treating pump in such a direction that oxygen was introduced intothe gas-measuring chamber. The self-heating-type heater was controlledby the signal of a thermocouple embedded in the gas-detecting device,such that the temperature of a detection region was 600° C.

[0248] The output of each sample was measured by a voltmeter with highinput impedance, and the sensitivity of the device was evaluated by thedifference between the output obtained in the case of a base gascontaining 5% by volume of oxygen, the balance being nitrogen, and theoutput obtained in the case of a detection gas containing 100 ppm of NOin addition to the base gas composition. Also, the interface impedancebetween the reference electrode or the oxygen-sensing electrode eachprovided with the electrode-coating layer and the solid electrolytesubstrate was measured by an impedance analyzer. The results are shownin Table 18. TABLE 17 Electrode-Coating Layer Laminate Porosity No.Sensor Structure Material Size (mm) (vol. %) Com. Ex. 19 No — — Example95⁽¹⁾ CeO₂ (12 mol %)-ZrO₂   1 × 1.8 × 0.01 30 Example 96⁽¹⁾ Y₂O₃ (8 mol%)-ZrO₂   1 × 1.8 × 0.01 30 Example 97⁽¹⁾ MgO (15 mol %)-ZrO₂   1 × 1.8× 0.01 30 Example 98⁽¹⁾ Sc₂O₃ (12 mol %)-ZrO₂   1 × 1.8 × 0.01 30Example 99⁽¹⁾ CeO₂ (12 mol %)-ZrO₂   1 × 1.8 × 0.01 10 Example 100⁽¹⁾CeO₂ (12 mol %)-ZrO₂   1 × 1.8 × 0.01 20 Example 101⁽¹⁾ CeO₂ (12 mol%)-ZrO₂   1 × 1.8 × 0.01 50 Example 102⁽¹⁾ CeO₂ (12 mol %)-ZrO₂   1 ×1.8 × 0.01 30 Example 103⁽¹⁾ CeO₂ (12 mol %)-ZrO₂   1 × 1.8 × 0.015 30Example 104⁽¹⁾ CeO₂ (12 mol %)-ZrO₂   1 × 1.8 × 0.02 30 Example 105⁽²⁾CeO₂ (12 mol %)-ZrO₂ 0.7 × 1.8 × 0.005  0 Example 106⁽²⁾ CeO₂ (12 mol%)-ZrO₂ 0.7 × 1.8 × 0.005  0 Example 107⁽²⁾ CeO₂ (12 mol %)-ZrO₂ 0.7 ×1.8 × 0.005  0 Example 108⁽²⁾ Y₂O₃ (3 mol %)-ZrO₂ 0.7 × 1.8 × 0.005  0Example 109⁽²⁾ Y₂O₃ (3 mol %)-ZrO₂ 0.7 × 1.8 × 0.005  0 Example 110⁽²⁾Y₂O₃ (3 mol %)-ZrO₂ 0.7 × 1.8 × 0.005  0

[0249] TABLE 18 Sensitivity to Interface 100 ppm of Impedance No. OxygenElectrode⁽³⁾ NO (mV) (kΩ) Com. Ex. 19 Pt 30 40 Example 95⁽¹⁾ Pt 42 20Example 96⁽¹⁾ Pt 41 22 Example 97⁽¹⁾ Pt 39 24 Example 98⁽¹⁾ Pt 40 23Example 99⁽¹⁾ Pt 43 19 Example 100⁽¹⁾ Pt 43 19 Example 101⁽¹⁾ Pt 38 22Example 102⁽¹⁾ Pt 39 24 Example 103⁽¹⁾ Pt 42 22 Example 104⁽¹⁾ Pt 42 22Example 105⁽²⁾ Pt 46 18 Example 106⁽²⁾ Pt 47 18 Example 107⁽²⁾ Pt 45 20Example 108⁽²⁾ Pt 45 19 Example 109⁽²⁾ Pt-50% by mass Ir 46 20 Example110⁽²⁾ Pt-1% by mass Au 43 23

[0250] In the sensor of Comparative Example 19 having a sensingelectrode 5 and a reference electrode 7 both not provided with anelectrode-coating layer as shown in FIG. 15, the sensitivity to 100 ppmof NO was 30 mV, and the interface impedance was 40 kΩ. On the otherhand, in the sensors of Examples each having a reference electrodelaminated with an electrode-coating layer, the interface impedance wasreduced to about 20 kΩ, and its sensitivity was as large as about 40 mVor more.

[0251] The reasons therefor are as follows:

[0252] (1) By laminating the electrode-coating layer 12, the referenceelectrode 7 was provided with decreased interface impedance, resultingin decrease in activity with NOx.

[0253] (2) Because Theobromine embedded in a portion of the laminatecorresponding to the gas-measuring chamber 4 in the lamination processis sublimed with rapid volume expansion at the degreasing step, Rh isnot transferred as a contamination component from the NOx-convertingelectrode 8 disposed at an position opposing the reference electrode 7because of the sensor structure to the reference electrode 7, so that noactivity with NOx is generated in the reference electrode 7.

[0254] (3) The lamination of an electrode-coating layer 12 prevents suchcontamination component from reaching the three-phase interface of thereference electrode 7, so that activity with NOx was not generated inthe reference electrode 7, resulting in increase in sensitivity.

[0255] Thus, by laminating the electrode-coating layer 12 made of anoxygen-ion-conductive solid electrolyte onto the reference electrode 7,the reference electrode 7 is not easily subjected to influence by theNOx concentration in the detection gas, thereby providing a sensorcapable of detecting the NOx concentration with high precision.

[0256] In Examples 95-98, the influence of materials, particularlyzirconia stabilizers, of the electrode-coating layers 12 of the sensorsshown in FIG. 15 was examined. The layer 12 had a size of 1 mm×1.8mm×0.01 mm and a porosity of 30% by volume, and the reference electrode7 was completely covered by the electrode-coating layer 12. Whateverstabilizers were added, the interface impedance decreased and thesensitivity increased by about 10 mV as compared with ComparativeExample 19. This confirmed that the activity of the reference electrode7 to NOx was decreased.

[0257] In Examples 95 and 99-101, the influence of the porosity of theelectrode-coating layers 12 in the sensors shown in FIG. 15 wasexamined. Each electrode-coating layer 12 had a size of 1 mm×1.8 mm×0.01mm and completely covered each reference electrode 7. Within this rangeof porosity, they had smaller interface impedance than that ofComparative Example 1, and larger sensitivity by about 10 mV than thatof Comparative Example 19. This confirmed that the activity of thereference electrode 7 to NOx was decreased.

[0258] In Examples 95 and 102-104, the influence of the thickness of theporous electrode-coating layer in the sensors shown in FIG. 15 wasexamined. Each electrode-coating layer 12 had an area of 1 mm×1.8 mm andcompletely covered the reference electrode 7. Within this range ofthickness, they had smaller interface impedance than that of ComparativeExample 19, and larger sensitivity by about 10 mV than that ofComparative Example 19. This confirmed that the activity of thereference electrode 7 to NOx was decreased.

[0259] In Example 105, the characteristics of the sensor shown in FIG.15 were examined, when the reference electrode 7 was laminated with anelectrode-coating layer 12 constituted by dense ceria-stabilizedzirconia, and when only a vertical side surface of the referenceelectrode 7 was not covered by the electrode-coating layer 12. Thesensors showed remarkably decreased interface impedance than that ofComparative Example 19, and sensitivity of 46 mV to 100 ppm of NO, 16 mVincrease than Comparative Example 19. This confirmed that the activityof the reference electrode 7 to NOx was decreased. Though it wassuspected that the sensors might show a decreased response speed becausethe coating layer 12 was dense, there was no substantial difference inresponse as compared with when the porous electrode-coating layer wasused.

[0260] In Examples 106 and 107, the effect of the coating layers 13 onthe oxygen-sensing electrode 6 in the sensors shown in FIGS. 17 and 18,respectively, was examined. Examples 106 and 107 showed the samesensitivity and interface impedance as those of Example 105. Thisconfirmed that the activity of the reference electrode 7 to NOx wasdecreased.

[0261] In Example 108, the characteristics of the sensor shown in FIG.18 were examined, when the oxygen-sensing electrode 6 was laminated withthe electrode-coating layer 13 constituted by dense zirconia stabilizedby 3 mol % of yttria, and when a vertical side surface of theoxygen-sensing electrode 6 was not covered by the electrode-coatinglayer 13. Other electrodes than the oxygen-sensing electrode 6 were notlaminated with electrode-coating layers. Example 108 showed the samesensitivity and interface impedance as those of Example 105. Thisconfirmed that the activity of the oxygen-sensing electrode 6 to NOx wasdecreased.

[0262] In Examples 109 and 110, the influence of materials of theoxygen-sensing electrode 6 of the sensor shown in FIG. 18 was examined,when the oxygen-sensing electrode 6 was laminated with theelectrode-coating layer 13 constituted by dense zirconia stabilized by 3mol % of yttria. The electrode-coating layer 13 did not cover a verticalside surface of the oxygen-sensing electrode 6. Other electrodes thanthe oxygen-sensing electrode 6 were not covered by an electrode-coatinglayer. The electrode made of an alloy of Pt and 50% by mass of Ir andthe oxygen-sensing electrode 6 made of an alloy of Pt and 1% by mass ofAu showed the same sensitivity and interface impedance as those ofExample 105. This confirmed that these alloys were usable for theoxygen-sensing electrode 6.

[0263] It was found that though sensitivity gradually decreased as theusing time elapsed in Comparative Example 19, the change of sensitivitywith time was remarkably suppressed in any Examples, in which thereference electrode 7 or the oxygen-sensing electrode 6 was covered byan electrode-coating layer made of an oxygen-ion-conductive solidelectrolyte. This is presumed to be due to the fact that theelectrode-coating layer suppressed contamination during operation,keeping the reference electrode 7 or the oxygen-sensing electrode 6 lowin activity with NOx.

EXAMPLES 111-116

[0264] Samples of sensor elements having the structure shown in FIG. 1were produced in the same manner as in Reference Example 1, except forusing materials shown in Table 19 (containing 10% by mass of a zirconiasolid electrolyte) for the sensing electrodes 5, using zirconiacontaining 10 mol % of CeO₂ (porosity: 30%, average thickness: 15 μm) orzirconia containing 12 mol % of SC₂O₃ (porosity: 30%, average thickness:15 μm) for the electrode-coating layers 11, and using Pt for thereference electrodes 7. With each of the resultant sensor samples set inan electric furnace, the detection performance to each detection gascontaining C₃H₆ (30 ppm), CO (20 ppm) or NH₃ (50 ppm) was evaluated inthe same manner as in Example 1. The results are shown in Table 19.

Comparative Examples 20-23

[0265] Sensor samples were produced to evaluate their detectionperformance in the same manner as in Examples 111-116 except that noelectrode-coating layer was formed. The results are shown in Table 19.TABLE 19 No. Sensing electrode⁽¹⁾ Electrode-Coating Layer Example 111NiCr₂O₄ CeO₂ (10 mol %)-ZrO₂ Example 112 NiCr₂O₄ CeO₂ (10 mol %)-ZrO₂Example 113 Pt-Rh (5% by mass) CeO₂ (10 mol %)-ZrO₂ Example 114 Cr₂O₃CeO₂ (10 mol %)-ZrO₂ Example 115 NiCr₂O₄ Sc₂O₃ (12 mol %)-ZrO₂ Example116 NiCr₂O₄ Sc₂O₃ (12 mol %)-ZrO₂ Com. Ex. 20 NiCr₂O₄ No Com. Ex. 21NiCr₂O₄ No Com. Ex. 22 Pt-Rh (5% by mass) No Com. Ex. 23 Cr₂O₃ NoInitial Sensitivity (mV) Change Ratio of C₃H₆ ⁽²⁾ CO⁽³⁾ NH₃ ⁽⁴⁾ Drift⁽⁵⁾(%) Example 111 −42 — — +24 Example 112 −50 — — +14 Example 113 — −40 —+27 Example 114 — — −42 −32 Example 115 −44 — — +20 Example 116 −53 — —+23 Com. Ex. 22 −46 — — +63 Com. Ex. 23 −58 — — +51 Com. Ex. 24 — −41 —+72 Com. Ex. 25 — — −49 −62

[0266] As shown in Table 19, Examples 111-116 showed smaller drift thanComparative Examples 22-25. This proved that the use of thegas-detecting element of the present invention greatly suppressed driftin any gas of HC (hydrocarbons), CO (carbon monoxide) and NH₃ (ammonia).

EXAMPLES 117-120 Comparative Example 24

[0267] Laminate-type NOx sensors having the structure shown in FIG. 21were produced by the following procedures. Zirconia powder containing 6mol % of yttria was used as starting material powder for zirconia solidelectrolyte substrates to form green sheets. Each green sheet had a sizeof 0.25 mm×5 mm×50 mm. Incidentally, when a sintered substrate is used,the thickness of a substrate is about 200 μm. Each green sheet was cutto a rectangular shape, and screen-printed with each sensor structureportion. The green sheets were laminated and pressed to provide alaminate structure. Each of the resultant laminate structures wasdegreased at 500° C. for 2 hours in air and then sintered at 1400° C.for 3 hours in air.

[0268] NiCr₂ O₄ was used for a NOx-sensing electrode 5 having a size of0.7 mm×1.3 mm×0.003 mm. A reference electrode 7 made of Pt was formed onthe zirconia solid electrolyte substrate 1 on the same surface as theNOx-sensing electrode 5 in the vicinity of the NOx-sensing electrode 5.The reference electrode 7 had a size of 0.7 mm×1.3 mm×0.003 mm, like theNOx-sensing electrode 5. Pt-3% by mass Rh was used for a NOx-convertingelectrode 8, and Pt was used for a conversion counter electrode 9. Anelectrode-coating film layer 51 of the NOx-converting electrode 8 wasprovided with a portion directly bonded to the solid electrolytesubstrate 2.

[0269] The electrode-coating film layer 51 of the NOx-convertingelectrode 8 was formed by a zirconia solid electrolyte containing astabilizer with various porosity and thickness. Produced in ComparativeExample 26 was a laminate-type NOx sensor having the same structure asin Example 117 except for having no electrode-coating film layer 51.

[0270] Each laminate-type NOx sensor was assembled in an apparatus forevaluating gas response characteristics, to carry out the followingevaluation on the effects of the electrode-coating layer 51. First, eachlaminate-type NOx sensor was stationarily operated at 600° C. in anitrogen gas (base gas) atmosphere containing 5% by volume of oxygen inthe evaluation apparatus, to measure the impedance of interface betweenthe conversion electrode and the solid electrolyte substrate at theinitial stage of operation and after predetermined time of operation,respectively. The stability of the conversion electrode 8 was evaluatedby the change ratio of the interface impedance.

[0271] A detection gas formulated by adding 100 ppm of NO to the basegas was supplied to each laminate-type NOx sensor at a temperature of600° C., to measure the sensor output (first output) to NO. Next, eachlaminate-type NOx sensor was stationarily operated at 600° C. forpredetermined time in a nitrogen gas (reducing gas) atmospherecontaining 10% of carbon monoxide (CO) and 5000 ppm of propane (C₃H₈),and then the detection gas was supplied to each laminate-type NOx sensorat a temperature of 600° C. to measure the sensor output (second output)to NO. The first output is called a sensor output (sensitivity) at theinitial stage of operation, and the second output is called a sensoroutput (sensitivity) after operation. The stability of the conversionelectrode 8 was evaluated by the change ratio from the sensor output atthe initial stage of operation to the sensor output after operation.

[0272] The stabilizers added to the conversion electrode-coating layer51, the thickness and porosity of the coating layer 51, the change ratioof interface impedance and the change ratio of sensor output(sensitivity) are shown in Table 20. TABLE 20 Electrode-Coating LayerFor Change Conversion Electrode Ratio of Change Thickness PorosityInterface Ratio of No. Material (μm) (vol. %) Impedance⁽¹⁾Sensitivity⁽²⁾ Com. No — — 30% −19% Ex. 24 Ex. 117 CeO₂ 10 30 3% −3% (12mol %)- ZrO₂ Ex. 118 Y₂O₃ 10 30 2% −3% (8 mol %)- ZrO₂ Ex. 119 MgO 10 306% −5% (15 mol %)- ZrO₂ Ex. 120 Sc₂O₃ 10 30 5% −6% (12 mol %)- ZrO₂

[0273] While the interface impedance increased by 30% in thelaminate-type NOx sensor of Comparative Example 26, in which theNOx-converting electrode-coating layer 51 was not formed, the changeratio of interface impedance was as small as 2-6% in the laminate-typeNOx sensors in Examples 117-120. This proved that the formation of theelectrode-coating layer 51 made of a zirconia solid electrolytecontaining ceria (CeO₂), yttria (Y₂O₃), magnesia (MgO) or scandia(Sc₂O₃) as a stabilizer on the conversion electrode 8 contributed to thestabilization of interface between the conversion electrode 8 and thesolid electrolyte substrate 2.

[0274] While the sensitivity to 100 ppm of NO decreased by 19% in thelaminate-type NOx sensor of Comparative Example 26, in which theelectrode-coating layer 51 was not formed on the NOx-convertingelectrode, the change ratio of sensitivity was as low as 3-6% in thelaminate-type NOx sensors in Examples 117-120. This proved that thesensitivity of the laminate-type NOx sensor was stabilized by formingthe electrode-coating layer 51 made of a zirconia solid electrolytecontaining a stabilizer on the conversion electrode 8.

EXAMPLES 121-123

[0275] Each laminate-type NOx sensor was produced and evaluated in thesame manner as in Example 117 except for changing the porosity of theconversion electrode-coating layer 51. The influence of the porosity ofthe conversion electrode-coating layer 51 on the change ratio ofinterface impedance and the change ratio of sensor output (sensitivity)was examined. The results are shown in Table 21. TABLE 21Electrode-Coating Layer Change for Conversion Electrode Ratio of ChangeThickness Porosity Interface Ratio of No. Material (μm) (vol. %)Impedance⁽¹⁾ Sensitivity⁽²⁾ Ex. 117 CeO₂ 10 30 3% −3% (12 mol %)- ZrO₂Ex. 121 CeO₂ 10 10 2% −2% (12 mol %)- ZrO₂ Ex. 122 CeO₂ 10 20 3% −2% (12mol %)- ZrO₂ Ex. 123 CeO₂ 10 50 4% −6% (12 mol %)- ZrO₂

[0276] A zirconia solid electrolyte stabilized by 12 mol % CeO₂ was usedfor the NOx-converting electrode-coating layer 51. By changing theporosity of the conversion electrode-coating layer 51 to 10-50%, thechange of interface impedance and sensor output (sensitivity) could besuppressed, thereby providing the laminate-type NOx sensors withexcellent stability.

EXAMPLES 124-126

[0277] Each laminate-type NOx sensor produced and evaluated in the samemanner as in Example 117 except for changing the thickness of theconversion electrode-coating layer 51. The influence of the thickness ofthe conversion electrode-coating layer 51 on the change ratio ofinterface impedance and the change ratio of sensor output (sensitivity)was examined. The results are shown in Table 22. TABLE 22Electrode-Coating Layer Change for Conversion Electrode Ratio of ChangeThickness Porosity Interface Ratio of No. Material (μm) (vol. %)Impedance⁽¹⁾ Sensitivity⁽²⁾ Ex. 117 CeO₂ 10 30 3% −3% (12 mol %)- ZrO₂Ex. 124 CeO₂ 3 30 5% −7% (12 mol %)- ZrO₂ Ex. 125 CeO₂ 15 30 3% −2% (12mol %)- ZrO₂ Ex. 126 CeO₂ 20 30 2% −2% (12 mol %)- ZrO₂

[0278] A zirconia solid electrolyte stabilized by 12 mol % of CeO₂ wasused for the NOx-converting electrode-coating layer 51. By regulatingthe thickness of the conversion electrode-coating layer 51 to 3-20 μm,the change of interface impedance and sensor output (sensitivity) couldbe suppressed, thereby providing the laminate-type NOx sensors withexcellent stability.

EXAMPLES 127-138

[0279] Each laminate-type NOx sensor produced and evaluated in the samemanner as in Example 117 except for adding various precious metalsand/or metal oxides to a zirconia solid electrolyte for the conversionelectrode-coating layer 51. The influence of the additives to theconversion electrode-coating layer 51 on the change ratio of interfaceimpedance and the change ratio of sensor output (sensitivity) wasexamined. The results are shown in Table 23. TABLE 23 Electrode-CoatingLayer Change for Conversion Electrode Ratio of Change Thickness PorosityInterface Ratio of No. Material (μm) (vol. %) Impedance⁽¹⁾Sensitivity⁽²⁾ Ex. 117 No 10 30 3% −3% Ex. 127 20 vol. % Rh 10 30 2% −2%Ex. 128 20 vol. % Ir 10 30 4% −2% Ex. 129  5 vol. % Au 10 30 5% −3% Ex.130 20 vol. % 10 30 3% −1% (Pt-3 wt. % Rh) Ex. 131 20 vol.% 10 30 3% −2%(Pt-10 wt. % Ir) Ex. 132 20 vol. % 10 30 4% −3% (Pt-3 wt. % Au) Ex. 13310 vol. % Cr₂O₃ 10 30 5% −2% Ex. 134 10 vol. % NiO 10 30 6% −2% Ex. 13510 vol. % NiCr₂O₄ 10 30 4% −2% Ex. 136 10 vol. % MgCr₂O₄ 10 30 3% −2%Ex. 137 10 vol. % FeCr₂O₄ 10 30 3% −2% Ex. 138 10 vol. % (Pt-3 wt. % 1030 2% −1% Rh) + 10 vol. % NiCr₂O₄

[0280] The change of interface impedance and sensor output (sensitivity)could be suppressed to provide the laminate-type NOx sensors withexcellent stability in Examples 127-138, in which precious metals and/ormetal oxides were added, like in Example 117, in which no additive wasadded to the conversion electrode-coating layer 51.

EXAMPLES 139

[0281] Laminate-type NOx sensors were produced in the same manner as inExample 117, except that electrode underlayers 52 (porosity: 10%,thickness: 3 μm) having various compositions were formed on theconversion electrodes 8 as shown in FIG. 22. In the structure shown inFIG. 22, the electrode-coating film layer 51 of the NOx-convertingelectrode 8 had a portion bonded to the solid electrolyte substrate 2via the electrode underlayer 52. The electrode underlayer 52 was basedon a zirconia solid electrolyte, which contained various stabilizersand/or additives as shown in Table 24. Incidentally, the amount of astabilizer is expressed by “mol %” based on the total amount (100 mol %)of a stabilized zirconia solid electrolyte, and the amount of anadditive is expressed by “% by volume” based on the total amount (100%by volume) of the electrode underlayer 52. Also, a zirconia solidelectrolyte stabilized by 12 mol % CeO₂ was used for the conversionelectrode-coating layer 51, and its porosity and thickness were 30% byvolume and 10 μm, respectively.

[0282] Laminate-type NOx sensors provided with various conversionelectrode underlayers 52 were measured with respect to interfaceimpedance and sensor output (sensitivity) in the same manner as inExample 117. The influence of the composition of the electrodeunderlayer 52 on the change ratio of interface impedance and the changeratio of sensor output (sensitivity) was examined. The results are shownin Table 24. TABLE 24 Electrode-Coating Layer Change Ratio Change forConversion Electrode of Interface Ratio of No. Structure MaterialImpedance⁽¹⁾ Sensitivity⁽²⁾ Ex. 117 No 3% −3% Ex. 139 CeO₂ (12 mol %)-3% −3% ZrO₂

[0283] The change of interface impedance and the change of sensor output(sensitivity) could be suppressed in the laminate-type NOx sensors eachhaving a electrode underlayer 52 in Examples 139-150, regardless of thecomposition of the electrode underlayer 52, the presence or absence of astabilizer and an additive and its type. As a result, the laminate-typeNOx sensors were provided with excellent stability.

[0284] Though the gas-detecting element and the gas-detecting device ofthe present invention have been explained above referring to thedrawings, it should be noted that the present invention is notrestricted thereto, and that various modifications can be made theretoas long as they do not change the spirit of the present invention.

[0285] By covering the electrodes fixed onto the oxygen-ion-conductivesolid electrolyte substrate with electrode-coating layers, theelectrochemical activity of electrode interface can be stabilized, andthe interface impedance of electrodes can be reduced. Accordingly, thegas-detecting element and gas-detecting device of the present inventionare improved in response performance during gas detecting, exhibitinghigh precision stably in the detection of the concentration of adetection object gas.

[0286] Also, by laminating the oxygen-ion-conductive electrode-coatinglayer, through which a detection object gas is diffusible, on theconversion electrode fixed onto the solid electrolyte substrate made ofa oxygen ion conductor, and adhering the electrode-coating layer to thesolid electrolyte substrate directly or via the electrode underlayermade of an oxygen-ion-conductive solid electrolyte, it is possible toalleviate thermal strain due to the difference in thermal expansion andsintering shrinkage, etc. between the conversion electrode and the solidelectrolyte, and it is possible to stabilize the electrochemicalactivity of the conversion electrode.

[0287] The gas-detecting element and the gas-detecting device of thepresent invention having the above features have high sensitivity andcan perform stable gas detection, particularly suitable for thedetection of NOx.

What is claimed is:
 1. A gas-detecting element comprising anoxygen-ion-conductive solid electrolyte substrate, a sensing electrodefixed onto said solid electrolyte substrate and active with a detectionobject gas and oxygen, and a reference electrode fixed onto said solidelectrolyte substrate and active with at least oxygen, to determine theconcentration of said detection object gas from the potential differencebetween said sensing electrode and said reference electrode, whereinsaid sensing electrode and/or said reference electrode is covered by anelectrode-coating layer made of an oxygen-ion-conductive solidelectrolyte, and wherein said electrode-coating layer has a portionbonded to said solid electrolyte substrate directly or via an electrodeunderlayer made of an oxygen-ion-conductive solid electrolyte.
 2. Agas-detecting element comprising an oxygen-ion-conductive solidelectrolyte substrate, a sensing electrode fixed onto said solidelectrolyte substrate and active with a detection object gas and oxygen,an oxygen-sensing electrode fixed onto said solid electrolyte substrateand active with at least oxygen, a reference electrode positioned in anatmosphere separated from a detection object atmosphere and active withoxygen, to determine the concentration of said detection object gas fromthe difference (E₁-E₂) between a potential difference E₁ between saidsensing electrode and said reference electrode and a potentialdifference E₂ between said oxygen-sensing electrode and said referenceelectrode, wherein said sensing electrode and/or said oxygen-sensingelectrode is covered by an electrode-coating layer made of anoxygen-ion-conductive solid electrolyte, and wherein saidelectrode-coating layer has a portion bonded to said solid electrolytesubstrate directly or via an electrode underlayer made of anoxygen-ion-conductive solid electrolyte.
 3. The gas-detecting elementaccording to claim 2, wherein said reference electrode is covered by anelectrode-coating layer made of an oxygen-ion-conductive solidelectrolyte, and wherein said electrode-coating layer has a portionbonded to said solid electrolyte substrate directly or via an electrodeunderlayer made of an oxygen-ion-conductive solid electrolyte.
 4. Thegas-detecting element according to claim 1, wherein at least one of saidsensing electrode, said reference electrode and said oxygen-sensingelectrode is fixed onto said solid electrolyte substrate via an electricinsulating layer.
 5. The gas-detecting element according to claim 1,wherein at least one of said sensing electrode, said reference electrodeand said oxygen-sensing electrode is fixed in a recess of said solidelectrolyte substrate.
 6. The gas-detecting element according to claim1, wherein said electrode-coating layer covering said sensing electrodehas a porosity of 10-50%.
 7. The gas-detecting element according toclaim 1, wherein said electrode-coating layer covering said sensingelectrode has an average thickness of 3-20 μm.
 8. The gas-detectingelement according to claim 1, wherein said electrode-coating layercovering said reference electrode or said oxygen-sensing electrode has aporosity of 0-50%.
 9. The gas-detecting element according to claim 1,wherein said electrode-coating layer covering said reference electrodeor said oxygen-sensing electrode has an average thickness of 1-20 μm.10. The gas-detecting element according to claim 1, wherein saidelectrode-coating layer covering at least one of said sensing electrode,said reference electrode and said oxygen-sensing electrode has anaverage thickness of 5-100 μm, and said electrode-coating layer hasgas-diffusing pores, a ratio (Sh/Se) of the total opening area (Sh) ofsaid gas-diffusing pores to the area (Se) of said sensing electrodebeing 0.05-0.28.
 11. The gas-detecting element according to claim 1,wherein an upper surface of at least one electrode of said referenceelectrode and said oxygen-sensing electrode exposed to a detection gasis covered by a dense electrode-coating layer, and part of side surfacesof said electrode is exposed.
 12. The gas-detecting element according toclaim 1, wherein a plurality of sensing electrodes are formed via saidelectrode-coating layer.
 13. The gas-detecting element according toclaim 1, wherein an electrode-coating layer covering at least one ofsaid sensing electrode, said reference electrode and said oxygen-sensingelectrode is made of a zirconia solid electrolyte containing as astabilizer at least one selected from the group consisting of yttria,ceria, magnesia and scandia.
 14. The gas-detecting element according toclaim 1, wherein an electrode-coating layer covering said sensingelectrode contains a precious metal active with said detection objectgas and oxygen.
 15. The gas-detecting element according to claim 1,wherein an electrode-coating layer covering at least one of said sensingelectrode, said reference electrode and said oxygen-sensing electrodecontain a precious metal inactive with said detection object gas butactive with oxygen.
 16. The gas-detecting element according to claim 1,wherein said electrode underlayer is made of a zirconia solidelectrolyte containing as a stabilizer at least one selected from thegroup consisting of yttria, ceria, magnesia and scandia.
 17. Agas-detecting device comprising (a) a gas-measuring chamber defined byfirst and second oxygen-ion-conductive solid electrolyte substratesdisposed with a predetermined gap; (b) a gas inlet provided so that adetection gas flows into said gas-measuring chamber with a predeterminedgas diffusion resistance; (c) a gas-detecting element comprising asensing electrode fixed onto said first solid electrolyte substrate suchthat it is exposed to an atmosphere in said gas-measuring chamber, andactive with a detection object gas and oxygen, and a reference electrodefixed onto said first solid electrolyte substrate and active with atleast oxygen; (d) a detection-object-gas-converting pump elementcomprising (i) a detection-object-gas-converting electrode fixed ontosaid second solid electrolyte substrate such that it is exposed to anatmosphere in said gas-measuring chamber, and active with a detectionobject gas and oxygen, and (ii) a detection-object-gas-convertingcounter electrode fixed onto said second solid electrolyte substratesuch that it is exposed to an atmosphere containing oxygen and/or anoxide gas, and active with oxygen, which can select the oxidation orreduction of a detection object gas depending on conditions; (e) a meansfor measuring the potential difference between said sensing electrodeand said reference electrode; and (f) a voltage-applying means fordriving said conversion pump element, to detect the potential differencebetween said sensing electrode and said reference electrode whileapplying predetermined voltage to said conversion pump element, therebydetermining the concentration of said detection object gas in saiddetection gas, wherein said sensing electrode and/or said referenceelectrode is covered by an electrode-coating layer made of anoxygen-ion-conductive solid electrolyte, and said electrode-coatinglayer has a portion bonded to said first solid electrolyte substratedirectly or via an electrode underlayer made of an oxygen-ion-conductivesolid electrolyte.
 18. The gas-detecting device according to claim 17,wherein said gas-detecting element comprises an oxygen-ion-conductivesolid electrolyte substrate, a sensing electrode fixed onto said solidelectrolyte substrate and active with a detection object gas and oxygen,and a reference electrode fixed onto said solid electrolyte substrateand active with at least oxygen, to determine the concentration of saiddetection object gas from the potential difference between said sensingelectrode and said reference electrode, wherein said sensing electrodeand/or said reference electrode is covered by an electrode-coating layermade of an oxygen-ion-conductive solid electrolyte, and wherein saidelectrode-coating layer has a portion bonded to said solid electrolytesubstrate directly or via an electrode underlayer made of anoxygen-ion-conductive solid electrolyte.
 19. The gas-detecting deviceaccording to claim 17, wherein said detection object gas is NOx.
 20. Agas-detecting device comprising (a) a gas-measuring chamber defined byfirst and second oxygen-ion-conductive solid electrolyte substratesdisposed with a predetermined gap; (b) a gas inlet provided so that adetection gas flows into said gas-measuring chamber with a predeterminedgas diffusion resistance; (c) a gas-detecting element comprising asensing electrode fixed onto said first solid electrolyte substrate suchthat it is exposed to an atmosphere in said gas-measuring chamber, andactive with a detection object gas and oxygen, and a reference electrodefixed onto said first solid electrolyte substrate and active with atleast oxygen; and (d) a detection-object-gas-converting pump elementcomprising (i) a detection-object-gas-converting electrode fixed ontosaid second solid electrolyte substrate such that it is exposed to anatmosphere in said gas-measuring chamber, and active with a detectionobject gas and oxygen, (ii) a detection-object-gas-converting counterelectrode fixed onto said second solid electrolyte substrate such thatit is exposed to an atmosphere containing oxygen and/or an oxide gas,and active with oxygen, which can select the oxidation or reduction of adetection object gas depending on conditions; (e) a means for measuringthe potential difference between said sensing electrode and saidreference electrode; and (f) a voltage-applying means for driving saidconversion pump element, thereby detecting the potential differencebetween said sensing electrode and said reference electrode whileapplying predetermined voltage to said conversion pump element, todetermine the concentration of said detection object gas in saiddetection gas; and wherein said detection-object-gas-convertingelectrode is covered by an electrode-coating layer made of anoxygen-ion-conductive solid electrolyte, through which said detectionobject gas can reach to said electrode; and said electrode-coating layerhas a portion bonded to said second solid electrolyte substrate directlyor via an electrode underlayer made of a solid electrolyte.